staging:iio:resolver:ad2s1210 cleanup gpio handling.

Basically make use of the gpio array functions.
Technically the free doesn't need as much info
as given here, but this keeps it clean and easy
to follow.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>