drivers: adc: stm32: Fix race condition with internal channels

When using one of the internal channels (die_temp, vbat, vref) the
channels are enabled in the individual drivers and disabled again
whenever an adc conversion is complete.

This creates a race condition if the ADC is used from multiple threads.

This commit moves the disabling of the channels to the individual
drivers.

Signed-off-by: Brian Juel Folkmann <bju@trackunit.com>