Cannot use isochronous endpoint with PluggableUSB.

When I defined an endpoint to be `EP_TYPE_ISOCHRONOUS_IN` in the **PluggableUSBModule** constructor, I noticed that the program will crash in function `USB_ISR` when calls function `UDD_InitEndpoints` at line [784](https://github.com/arduino/ArduinoCore-sam/blob/3394937e5df00636abc5823446ecba5aa6940e42/cores/arduino/USB/USBCore.cpp#L776). However, in the definition of [EP_TYPE_ISOCHRONOUS_IN](https://github.com/arduino/ArduinoCore-sam/blob/3394937e5df00636abc5823446ecba5aa6940e42/system/libsam/include/uotghs_device.h#L86) if I change **UOTGHS_DEVEPTCFG_EPSIZE_1024_BYTE** to **UOTGHS_DEVEPTCFG_EPSIZE_512_BYTE** then the program runs fine. I could not figure out why.

Start in cores/arduino/USB/USBCore.cpp at USB_ISR and the call to UDD_InitEndpoints around line 784. Compare the EP_TYPE_ISOCHRONOUS_IN definition in system/libsam/include/uotghs_device.h, then reproduce the crash with a PluggableUSBModule endpoint using the 1024-byte setting. Done means the isochronous endpoint initializes and the program no longer crashes.

Written by the indexing model from the issue text.