staging:iio:resolver rename ad2s120x ->ad2s1200

There are far too many possible part numbers that are
not covered in here to have such a generic name.
So move to naming after one of the supported parts.

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>