staging:iio:cdc:ade7746 replace mlock with driver private lock

The IIO subsystem is redefining iio_dev mlock to be used by IIO core
only for protecting device operating mode changes.

In driver ad7746 wherever mlock was used to protect hardware state
changes, it has been replaced with a driver private lock.

Signed-off-by: Aishwarya Pant <aishpant@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>