drivers: adc: adc_sam0: Fix interpretation of channels

Previously this was expected to be equal to 1 at all times. This doesn't
play well with the sample or other users (e.g: adc_shell). Instead, we
should count the number of active channels in the bitfield, and ensure
that only one is identified.

Signed-off-by: Attie Grande <attie.grande@argentum-systems.co.uk>