Warnings when compiling 1.4.1 release

With the 1.4.1 release there are numerous compilation warnings. I think all of this compiled cleanly in previous release. I'm throwing all this in one report as all showed up together. Some are benign, others could be real bugs - assigning a value to a const, returning a value from a void method, etc.

```text
.../Library/Arduino15/packages/arduino/hardware/renesas_uno/1.4.1/libraries/WDT/src/WDT.cpp: In constructor 'WDTimer::WDTimer()':
.../Library/Arduino15/packages/arduino/hardware/renesas_uno/1.4.1/libraries/WDT/src/WDT.cpp:25:14: warning: missing initializer for member 'st_wdt_instance_ctrl::p_context' [-Wmissing-field-initializers]
, _timeout {0}
^
.../Library/Arduino15/packages/arduino/hardware/renesas_uno/1.4.1/libraries/WDT/src/WDT.cpp:25:14: warning: missing initializer for member 'st_wdt_instance_ctrl::p_callback' [-Wmissing-field-initializers]
.../Library/Arduino15/packages/arduino/hardware/renesas_uno/1.4.1/libraries/WDT/src/WDT.cpp:25:14: warning: missing initializer for member 'st_wdt_instance_ctrl::p_callback_memory' [-Wmissing-field-initializers]
```

```text
.../Library/Arduino15/packages/arduino/hardware/renesas_uno/1.4.1/cores/arduino/IRQManager.cpp: In member function 'bool IRQManager::addPeripheral(Peripheral_t, void*)':
.../Library/Arduino15/packages/arduino/hardware/renesas_uno/1.4.1/cores/arduino/IRQManager.cpp:385:38: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
if (irqcfg->first_irq_number != FSP_INVALID_VECTOR) {
.../Library/Arduino15/packages/arduino/hardware/renesas_uno/1.4.1/cores/arduino/IRQManager.cpp:448:38: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
if (irqcfg->first_irq_number != FSP_INVALID_VECTOR) {
.../Library/Arduino15/packages/arduino/hardware/renesas_uno/1.4.1/cores/arduino/IRQManager.cpp:609:26: warning: unused variable 'scfg' [-Wunused-variable]
i2c_slave_cfg_t *scfg = (i2c_slave_cfg_t *)p_cfg->scfg;
^~~~
.../Library/Arduino15/packages/arduino/hardware/renesas_uno/1.4.1/cores/arduino/IRQManager.cpp:701:27: warning: unused variable 'mcfg' [-Wunused-variable]
i2c_master_cfg_t *mcfg = (i2c_master_cfg_t *)p_cfg->mcfg;
^~~~
.../Library/Arduino15/packages/arduino/hardware/renesas_uno/1.4.1/cores/arduino/IRQManager.cpp: In member function 'void IRQManager::set_canfd_error_link_event(int, int)':
.../Library/Arduino15/packages/arduino/hardware/renesas_uno/1.4.1/cores/arduino/IRQManager.cpp:1779:49: warning: unused parameter 'li' [-Wunused-parameter]
void IRQManager::set_canfd_error_link_event(int li, int ch)
^~
.../Library/Arduino15/packages/arduino/hardware/renesas_uno/1.4.1/cores/arduino/IRQManager.cpp:1779:57: warning: unused parameter 'ch' [-Wunused-parameter]
void IRQManager::set_canfd_error_link_event(int li, int ch)
^~
.../Library/Arduino15/packages/arduino/hardware/renesas_uno/1.4.1/cores/arduino/IRQManager.cpp: In member function 'void IRQManager::set_canfd_rx_link_event(int, int)':
.../Library/Arduino15/packages/arduino/hardware/renesas_uno/1.4.1/cores/arduino/IRQManager.cpp:1794:46: warning: unused parameter 'li' [-Wunused-parameter]
void IRQManager::set_canfd_rx_link_event(int li, int ch)
^~
.../Library/Arduino15/packages/arduino/hardware/renesas_uno/1.4.1/cores/arduino/IRQManager.cpp:1794:54: warning: unused parameter 'ch' [-Wunused-parameter]
void IRQManager::set_canfd_rx_link_event(int li, int ch)
^~
.../Library/Arduino15/packages/arduino/hardware/renesas_uno/1.4.1/cores/arduino/IRQManager.cpp: In member function 'void IRQManager::set_canfd_tx_link_event(int, int)':
.../Library/Arduino15/packages/arduino/hardware/renesas_uno/1.4.1/cores/arduino/IRQManager.cpp:1809:46: warning: unused parameter 'li' [-Wunused-parameter]
void IRQManager::set_canfd_tx_link_event(int li, int ch)
^~
.../Library/Arduino15/packages/arduino/hardware/renesas_uno/1.4.1/cores/arduino/IRQManager.cpp:1809:54: warning: unused parameter 'ch' [-Wunused-parameter]
void IRQManager::set_canfd_tx_link_event(int li, int ch)
^~
```

```text
.../Library/Arduino15/packages/arduino/hardware/renesas_uno/1.4.1/cores/arduino/cm_backtrace/cm_backtrace.c: In function 'cm_backtrace_init':
.../Library/Arduino15/packages/arduino/hardware/renesas_uno/1.4.1/cores/arduino/cm_backtrace/cm_backtrace.c:144:13: warning: assignment discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
fw_name = firmware_name;
^
.../Library/Arduino15/packages/arduino/hardware/renesas_uno/1.4.1/cores/arduino/cm_backtrace/cm_backtrace.c:145:12: warning: assignment discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
hw_ver = hardware_ver;
^
.../Library/Arduino15/packages/arduino/hardware/renesas_uno/1.4.1/cores/arduino/cm_backtrace/cm_backtrace.c:146:12: warning: assignment discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
sw_ver = software_ver;
^
```

```text
.../Library/Arduino15/packages/arduino/hardware/renesas_uno/1.4.1/cores/arduino/tinyusb/rusb2/dcd_rusb2.c: In function 'wait_pipe_fifo_empty':
.../Library/Arduino15/packages/arduino/hardware/renesas_uno/1.4.1/cores/arduino/tinyusb/rusb2/dcd_rusb2.c:289:3: warning: 'return' with a value, in function returning void
TU_ASSERT(num);
^~~~~~~~~
.../Library/Arduino15/packages/arduino/hardware/renesas_uno/1.4.1/cores/arduino/tinyusb/rusb2/dcd_rusb2.c:288:13: note: declared here
static void wait_pipe_fifo_empty(rusb2_reg_t* rusb, uint8_t num) {
^~~~~~~~~~~~~~~~~~~~
```

Start by reproducing a 1.4.1 compilation and compare its warnings with the previous release. Investigate WDT/src/WDT.cpp, cores/arduino/IRQManager.cpp, cm_backtrace/cm_backtrace.c, and tinyusb/rusb2/dcd_rusb2.c; done means determining whether each warning is benign or a real defect and producing a clean compilation where appropriate.

Written by the indexing model from the issue text.