drivers: adc: stm32g0 fix sampling time due to errata sheet

According to the ES0418 about the ADC of the stm32G071
or other stm32g0x devices:
for sampling time set to 1.5 or 3.5 cycles,
the sampling in a single ADC conversion or in the first
conversion of a sequence takes one extra cycle.
Minimizing to 7.5 is fine.

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