staging: iio: ad7280: Replace mlock with driver private lock

The IIO subsystem is redefining iio_dev->mlock to be used by
the IIO core only for protecting device operating mode changes.
ie. Changes between INDIO_DIRECT_MODE, INDIO_BUFFER_* modes.

In this driver, mlock was being used to protect hardware state
changes.  Replace it with a lock in the devices global data.

Signed-off-by: Gargi Sharma <gs051095@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>