iio: proximity: sx9500: claim direct mode during raw proximity reads

Driver was checking for direct mode but not locking it.  Use the
claim/release helper functions to guarantee the device stays in
direct mode during raw reads of proximity data.

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