SPI driver seems to hang at high clock frequencies

When testing SPI at a DIV2 clock divider (50 MHz, see https://github.com/oxidecomputer/hardware-cosmo/issues/766#issuecomment-3050259583), we noticed what looks like a hang in the SPI driver. Normally, cosmo_seq takes a tiny amount of time to configure FPGA2 over SPI3, but in this test it never finished:

➜  hubris git:(master) ✗ humility tasks
humility: attached via ST-Link V3
system time = 43495
ID TASK                       GEN PRI STATE
 0 jefe                         0   0 recv, notif: fault timer(T+5)
 1 net                          0   5 notif: jefe-state-change
 2 sys                          0   1 recv, notif: exti-wildcard-irq(irq6/irq7/irq8/irq9/irq10/irq23/irq40)
 3 spi2_driver                  0   3 recv
 4 spi3_driver                  0   3 notif: spi-irq(irq51)
 5 i2c_driver                   0   3 recv
 6 packrat                      0   1 recv
 7 thermal                      0   8 wait: send to cosmo_seq/gen0
 8 power                        0   8 wait: send to cosmo_seq/gen0
 9 hiffy                        0   7 notif: bit31(T+15)
10 cosmo_seq                    0   7 wait: reply from spi3_driver/gen0
11 ignition_flash               0   7 recv
12 spartan7_loader              0   4 recv
13 hash_driver                  0   2 recv
14 hf                           0   6 recv, notif: timer
15 update_server                0   3 recv
16 sensor                       0   4 recv
17 host_sp_comms                0   9 recv, notif: jefe-state-change usart-irq(irq71) multitimer control-plane-agent
18 udpecho                      0   6 wait: send to net/gen0
19 udpbroadcast                 0   6 wait: send to net/gen0
20 control_plane_agent          0   8 wait: send to net/gen0
21 sprot                        0   4 recv
22 validate                     0   5 recv
23 vpd                          0   4 recv
24 user_leds                    0   2 recv, notif: timer
25 dump_agent                   0   6 wait: send to net/gen0
26 sbrmi                        0   4 recv
27 spd                          0   7 notif: jefe-state-change
28 auxflash                     0   3 recv
29 idle                         0  15 RUNNING
30 udprpc                       0   6 wait: send to net/gen0
31 fmc_demo                     0   6 recv, notif: socket

humility tasks -sl spi3_driver
humility: attached via ST-Link V3
system time = 183694
ID TASK                       GEN PRI STATE
 4 spi3_driver                  0   3 notif: spi-irq(irq51)
   |
   +--->  0x24068290 0x0804c178 userlib::sys_recv_stub
                     @ /hubris/sys/userlib/src/lib.rs:368
          0x24068368 0x0804aa8a userlib::sys_recv
                     @ /hubris/sys/userlib/src/lib.rs:296
          0x24068368 0x0804aa8a userlib::sys_recv_notification
                     @ /hubris/sys/userlib/src/lib.rs:340
          0x24068368 0x0804a8f2 <drv_stm32h7_spi_server::ServerImpl as drv_stm32h7_spi_server::InOrderSpiImpl>::write
                     @ /hubris/drv/stm32h7-spi-server/src/main.rs:73
          0x24068368 0x0804a886 drv_stm32h7_spi_server::<impl idol_runtime::Server<drv_stm32h7_spi_server::SpiOperation> for (core::marker::PhantomData<drv_stm32h7_spi_server::SpiOperation>,&mut S)>::handle
                     @ /hubris/target/thumbv7em-none-eabihf/release/build/drv-stm32h7-spi-server-d0f0d84fec042ad1/out/server_stub.rs:243
          0x24068368 0x0804a63e idol_runtime::dispatch
                     @ /git/idolatry-1ebf1c2fd2f30300/6c54f3b/runtime/src/lib.rs:228
          0x24068368 0x0804aa9a main
                     @ /hubris/drv/stm32h7-spi-server/src/main.rs:33

Start in drv/stm32h7-spi-server/src/main.rs, especially InOrderSpiImpl::write at line 73, and inspect the stack shown for sys_recv_notification. Reproduce the DIV2, 50 MHz configuration test and use humility tasks to observe spi3_driver and cosmo_seq. Done means SPI3 configuration completes without leaving cosmo_seq waiting indefinitely.