stm32/adc: Increase sample time for internal sensors on L4 MCUs.

They need time (around 4us for VREFINT) to obtain accurate results.

Fixes issue #4022.

ports/stm32/adc.c

@@ -304,7 +304,13 @@ STATIC void adc_config_channel(ADC_HandleTypeDef *adc_handle, uint32_t channel)
     sConfig.OffsetRightShift = DISABLE;
     sConfig.OffsetSignedSaturation = DISABLE;
 #elif defined(STM32L4)
-    sConfig.SamplingTime = ADC_SAMPLETIME_12CYCLES_5;
+    if (channel == ADC_CHANNEL_VREFINT
+        || channel == ADC_CHANNEL_TEMPSENSOR
+        || channel == ADC_CHANNEL_VBAT) {
+        sConfig.SamplingTime = ADC_SAMPLETIME_247CYCLES_5;
+    } else {
+        sConfig.SamplingTime = ADC_SAMPLETIME_12CYCLES_5;
+    }
     sConfig.SingleDiff = ADC_SINGLE_ENDED;
     sConfig.OffsetNumber = ADC_OFFSET_NONE;
     sConfig.Offset = 0;