drivers: adc: stm32: suppress stm32h7 build warning

There is a build warning of unused `adc_stm32_oversampling_ratioshift`
function: [-Werror=unused-function]. This makes twister fail, as all
warnings are treated as errors.

Fix that by ifndef'ing adc_stm32_oversampling_ratioshift().

Fixes: c57a41c5 ("drivers/adc: stm32: do not disable the ADC if
  resolution is unchanged")
Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>