iio: magnetometer: mag3110: claim direct mode during raw writes

[ Upstream commit 80dea21f95a4672cce545f48dc2ca500b69a2584 ]

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

Signed-off-by: Alison Schofield <amsfield22@gmail.com>
Acked-by: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>