iio: adc: ad799x: use iio helper function to guarantee direct mode

Replace the code that guarantees the device stays in direct mode
with iio_device_claim_direct_mode() which does same.

Signed-off-by: Alison Schofield <amsfield22@gmail.com>
Acked-by: Daniel Baluta <daniel.baluta@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>