drivers: adc: stm32h7 adc driver has a different oversampling ratio

The adc_stm32_oversampling function is used to configure
the ratio and shift for each sequence->oversampling
depending on the soc serie and ADC instance in the serie
In the stm32H7 serie, only ADC3 of ADC_VER_V5_V90 version
have a LL_ADC_OVS_RATIO_x contant (other a 9bit value).

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