staging: iio: ad9834: Use private driver lock instead of mlock

iio_dev->mlock should be used by the IIO core only for protecting
device operating mode changes. ie. Changes between INDIO_DIRECT_MODE,
INDIO_BUFFER_* modes.
Replace mlock with a lock in the device's global data to protect
hardware state changes.

Signed-off-by: sayli karnik <karniksayli1995@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>