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

On those STM32 series, setting of this feature is conditioned to
the ADC state: it is allowed to set/reset the oversampler (OVSE bit)
and set the Oversampling ratio (OVSR bits) in the ADC_CFGR2 register
only when ADC is disabled or enabled without conversion on going.
Then is the ADC re-enabled.

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