drivers: adc: stm32: reset acquisition time between reads

For all STM32 ADC that use common sampling times, there is a check made to
ensure that all channels of a sequence use the same sampling time.
The value was not reset between reads, resulting in error if two
consecutive sequences used different values.
This commit adds a reset of this value once read is done.

Signed-off-by: Guillaume Gautier <guillaume.gautier-ext@st.com>