iio: adc: at91_adc: fix errors reported by checkpatch.pl

This fixes the errors reported by checkpatch.pl:

ERROR: space prohibited before that ',' (ctx:WxW)
ERROR: code indent should use tabs where possible

Signed-off-by: Slawomir Stepien <sst@poczta.fm>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>