Fix USB priority preventing CAN TX from timing out

[jorticus]

The blocking CAN implementation relies on SysTick for its timeout. USB had a higher priority than Systick, so the TX never timed out, causing the serial port to hang indefinitely.

[jorticus]

The latest master has an issue where trying to transmit on a bus with nothing listening will cause the adapter to completely hang. Even attempts to close the serial port from the OS hang, and you have to unplug the adapter from the USB port to recover.

I attached a SWD debugger to the device and found it was getting stuck in stm32f0xx_hal_can.c : HAL_CAN_Transmit() at while(!(__HAL_CAN_TRANSMIT_STATUS(hcan, transmitmailbox))). Further investigation showed that HAL_GetTick() wasn't incrementing its counter, which I found was because the interrupt wasn't being fired.

I'd suggest moving any blocking logic out of the ISR and into the main loop, but for now this PR should fix it!