drivers: adc: driver setting the oversampling for stm32wl

RM 0453: the sw is allowed to write the Oversampling
ratio or shift of the ADC Config.Reg.2 only when ADSTART = 0
(no conversion is on-going). So disabling it will be stopped.

Signed-off-by: Francois Ramu <francois.ramu@st.com>