iio: amplifiers: ad8366: Use right order for type specification

This patch fixes the following checkpatch.pl warning:
WARNING: type 'char unsigned' should be specified in [[un]signed]
[short|int|long|long long] order

Signed-off-by: Roberta Dobrescu <roberta.dobrescu@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>