fix: race condition on mixer task handle

PR #3451 introduced a change that caused the mixer task to be started out of the UI task, which introduces a race condition `mixerTaskId.rtos_handle`, as the mixer scheduler might trigger before `xTaskCreateStatic` returns and the task handle is assigned.

This PR fixes the issue by delaying the start of the mixer scheduler until `xTaskCreateStatic` returned and the task handler has been properly assigned.

radio/src/tasks/mixer_task.cpp

@@ -60,9 +60,11 @@ void mixerTaskUnlock()
 
 void mixerTaskInit()
 {
+  mixerSchedulerInit();
   RTOS_CREATE_MUTEX(mixerMutex);
   RTOS_CREATE_TASK(mixerTaskId, mixerTask, "mixer", mixerStack,
                    MIXER_STACK_SIZE, MIXER_TASK_PRIO);
+  mixerSchedulerStart();
 }
 
 bool mixerTaskStarted()
@@ -147,9 +149,6 @@ TASK_FUNCTION(mixerTask)
   gyroInit();
 #endif
 
-  mixerSchedulerInit();
-  mixerSchedulerStart();
-
   while (!_mixer_exit) {
 
     int timeout = 0;