stm32/adc: Fix reading internal ADC channels on L4 MCUs.

For STM32L4 series, the internal sensors are connected to:
- ADC1_IN0: Internal voltage reference
- ADC1_IN17: Temperature sensor
- ADC1_IN18: VBAT battery voltage monitoring
but ADC_CHANNEL_VREFINT, ADC_CHANNEL_VBAT, ADC_CHANNEL_TEMPSENSOR are not
defined as 0, 17, 18.

This commit converts channel 0, 17, 18 to ADC_CHANNEL_x in
adc_get_internal_channel().

ports/stm32/adc.c

@@ -216,6 +216,14 @@ static inline uint32_t adc_get_internal_channel(uint32_t channel) {
     if (channel == 16) {
         channel = ADC_CHANNEL_TEMPSENSOR;
     }
+    #elif defined(STM32L4)
+    if (channel == 0) {
+        channel = ADC_CHANNEL_VREFINT;
+    } else if (channel == 17) {
+        channel = ADC_CHANNEL_TEMPSENSOR;
+    } else if (channel == 18) {
+        channel = ADC_CHANNEL_VBAT;
+    }
     #endif
     return channel;
 }