staging: iio: adc: ad7192: use driver private lock to protect hardware state changes

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 driver private lock.

Also, as there are state changes in the ad7192_ write_raw function, a lock
is added to prevent the concurrent state changes.

Signed-off-by: Aastha Gupta <aastha.gupta4104@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>