iio: light: isl29125: 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. This allows
removal of an unused lock in the device private global data.
Signed-off-by: Alison Schofield <amsfield22@gmail.com> Cc: Daniel Baluta <daniel.baluta@gmail.com> Acked-by: Peter Meerwald-Stadler <pmeerw@pmeerw.net> Signed-off-by: Jonathan Cameron <jic23@kernel.org>