Fix USB priority preventing CAN TX from timing out

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.

src/usbd_conf.c

@@ -103,7 +103,8 @@ void HAL_PCD_MspInit(PCD_HandleTypeDef* pcdHandle)
     __HAL_RCC_USB_CLK_ENABLE();
 
     /* Peripheral interrupt init */
-    HAL_NVIC_SetPriority(USB_IRQn, 0, 0);
+    // IMPORTANT: USB must have a lower priority than SysTick, or timeouts won't work
+    HAL_NVIC_SetPriority(USB_IRQn, 1, 0);
     HAL_NVIC_EnableIRQ(USB_IRQn);
   /* USER CODE BEGIN USB_MspInit 1 */