drivers: sensor: stm32 Vref sensor calibration on 12bit

The Calibration value of the VRef on stm32U5 is acquired on 14Bit by ADC1
and should be adjusted on 12bit becasue the resolution is 12bit
in this stm32_vref driver.

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