staging: iio: ad7192: use iio_device_{claim|release}_direct_mode()

Replace the code that guarantees the device stays in direct mode with
iio_device_{claim|release}_direct_mode() which does same.

Signed-off-by: Alison Schofield <amsfield22@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>