AP_HAL_ChibiOS: add semaphore to AnalogIn::channel to make it thread safe

libraries/AP_HAL_ChibiOS/AnalogIn.cpp

@@ -357,6 +357,7 @@ void AnalogIn::_timer_tick(void)
 
 AP_HAL::AnalogSource* AnalogIn::channel(int16_t pin)
 {
+    WITH_SEMAPHORE(_semaphore);
     for (uint8_t j=0; j<ANALOG_MAX_CHANNELS; j++) {
         if (_channels[j] == nullptr) {
             _channels[j] = new AnalogSource(pin);

libraries/AP_HAL_ChibiOS/AnalogIn.h

@@ -88,6 +88,7 @@ private:
     static adcsample_t *samples;
     static uint32_t sample_sum[];
     static uint32_t sample_count;
+    HAL_Semaphore _semaphore;
 };
 
 #endif // HAL_USE_ADC