drivers/adc/adc_stm32: Check both single-ended and differential defines

The stm32u3x header files defines LL_ADC_SINGLE_ENDED but not
LL_ADC_DIFFERENTIAL as the device doesn't support differential mode. The
driver only checked for LL_ADC_SINGLE_ENDED and assumed that when that was
defined, LL_ADC_DIFFERENTIAL would also be defined.

Check for both when figuring out which calibration type will be required.

Signed-off-by: Keith Packard <keithp@keithp.com>