samples: adc_sequence: differential mode handling in ADC conversion

This commit updates the ADC sequence sample to improve handling of
differential mode readings. It modifies the conversion logic to
correctly interpret 16-bit/32-bit signed values and adjusts the
resolution accordingly. This ensures accurate millivolt conversion
for both differential and single-ended configurations.

Signed-off-by: Martin Hoff <martin.hoff@silabs.com>