staging: iio: ade7753: replace mlock with driver private lock

Replace driver usage of mlock with driver private lock to meet the new
model where usage of iio_dev->mlock is being redefined as protecting
operating mode changes(changes between BUFFER* and DIRECT modes).

Signed-off-by: Himanshi Jain <himshijain.hj@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>