iio: adc: ad7949: use devm managed functions

Switch to devm_iio_device_register to finalize devm migration.
This removes the use for iio_device_unregister() and since
mutex_destroy() is not necessary here, remove it altogether.

Signed-off-by: Liam Beguin <lvb@xiphos.com>
Link: https://lore.kernel.org/r/20210815213309.2847711-6-liambeguin@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>