staging: iio: ad5933: Protect DIRECT mode using claim/release helpers

This device operates in DIRECT_MODE and BUFFER_HARDWARE mode.
Replace usages of iio_dev->mlock with iio_device_{claim|release}_direct_mode()
helper functions to guarantee DIRECT mode and consequently protect
BUFFER mode too.

Add and use a device private lock to protect against conflicting access of the
state data.
This helps with IIO subsystem 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.

Protect changing of attributes inside ad5933_store(). Attributes
can no longer be changed while in buffered mode.

Remove lock from ad5933_work() because buffer mode should be enabled
when we reach this, and claiming DIRECT mode in all the other places
should protect it.

Signed-off-by: Narcisa Ana Maria Vasile <narcisaanamaria12@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>