STM32: ADC warning fix (#13933)

Marlin/src/HAL/HAL_STM32F1/HAL.cpp

 void HAL_adc_start_conversion(const uint8_t adc_pin) {
   TEMP_PINS pin_index;
   switch (adc_pin) {
+    default: return;
     #if HAS_TEMP_ADC_0
       case TEMP_0_PIN: pin_index = TEMP_0; break;
     #endif
   HAL_adc_result = (HAL_adc_results[(int)pin_index] >> 2) & 0x3FF; // shift to get 10 bits only.
 }
 
-uint16_t HAL_adc_get_result(void) {
-  return HAL_adc_result;
-}
+uint16_t HAL_adc_get_result(void) { return HAL_adc_result; }
 
 #endif // __STM32F1__