Fix USB crashes in FreeRTOS (#1419)

Fixes #1402

The global USB mutex is auto-shadowed with a FreeRTOS semaphore while in
FreeRTOS mode.  Unfortunately, while the core was using the proper
FreeRTOS semaphore to lock access to the USB port, the actual FreeRTOS
USB task was using the naked Pico SDK mutex, leading to cases where it
could acquire the mutex even though some other FreeRTOS task actually
owned the shadowed mutex.

Properly lock the shadowed Semaphore, not mutex_t, in the FreeRTOS
USB periodic task.

cores/rp2040/CoreMutex.cpp

@@ -33,7 +33,9 @@ CoreMutex::CoreMutex(mutex_t *mutex, uint8_t option) {
         if (_option & FromISR) {
             __freertos_mutex_take_from_isr(m);
         } else {
-            __freertos_mutex_take(m);
+            if (!__freertos_mutex_try_take(m)) {
+                return;
+            }
         }
     } else {
         uint32_t owner;

libraries/FreeRTOS/src/variantHooks.cpp

@@ -380,9 +380,10 @@ static void __usb(void *param) {
     __usbInitted = true;
 
     while (true) {
-        if (mutex_try_enter(&__usb_mutex, NULL)) {
+        auto m = __get_freertos_mutex_for_ptr(&__usb_mutex);
+        if (xSemaphoreTake(m, 0)) {
             tud_task();
-            mutex_exit(&__usb_mutex);
+            xSemaphoreGive(m);
         }
         vTaskDelay(1 / portTICK_PERIOD_MS);
     }