drivers: sensor: stm32_temp: setup channel before adc_read

Currently the driver only setup the ADC to read from the
internal temperature channel on init. However, it is possible
that some other application that uses the ADC can setup the
ADC to read from some other channel and therefore subsequent
stm32_temp_sample_fetch will fail to read the targeted channel.

Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>