iio: adc: ad7266: claim direct mode during sensor read

Driver was checking for direct mode but not locking it down.
Use iio_device_claim_direct_mode() to guarantee device stays
in direct mode.

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