For STM32G4 series, the internal sensors are connected to:
- ADC1_IN16: Temperature sensor
- ADC1_IN17: Battery voltage monitoring
- ADC1_IN18: Internal voltage reference
but ADC_CHANNEL_TEMPSENSOR_ADC1, ADC_CHANNEL_VBAT,
ADC_CHANNEL_VREFINT are not defined as 16, 17, 18.

This commit converts channel 16, 17, 18 to ADC_CHANNEL_x in
adc_get_internal_channel().
Signed-off-by: NYuuki NAGAO <wf.yn386@gmail.com>