staging: iio: Change GAIN macros to SCALE.

Per email exchange with Jonathan Cameron, GAIN should instead be SCALE, to
be consistent with documentation.

Resending, since this goes with the 2/2 patch.

Signed-off-by: Andrew Chew <achew@nvidia.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>