iio: dac: ad5592r-base: Replace indio_dev->mlock with own device lock

[ Upstream commit 33c53cbf ]

As part of the general cleanup of indio_dev->mlock, this change replaces
it with a local lock on the device's state structure.
This also removes unused iio_dev pointers.

Signed-off-by: Sergiu Cuciurean <sergiu.cuciurean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Stable-dep-of: 279428df ("iio: dac: ad5592r: fix temperature channel scaling value")
Signed-off-by: Sasha Levin <sashal@kernel.org>