drivers: adc: stm32 disable the ADC before setting the Resolution

On those STM32 series, setting of this feature is conditioned to
the ADC state: it is allowed to write the Data resolution (RES bits)
in the ADC_CFGR1 register only when ADC is disabled (ADEN=0).

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