iio: light: ltr501: claim direct mode during raw writes

Driver was checking for direct mode but not locking it.  Use
claim/release helper functions to guarantee the device stays
in direct mode during all raw write operations.

Signed-off-by: Alison Schofield <amsfield22@gmail.com>
Acked-by: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>