iio: adc: ad7923: register device with devm_iio_device_register

Registers the device using the devm variant.
This is the final step of converting the ad7923 to only use devm routines,
meaning that the ad7923_remove() function is no longer needed to release
resources on device detach.

Signed-off-by: Lucas Stankus <lucas.p.stankus@gmail.com>
Reviewed-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Link: https://lore.kernel.org/r/b0146465d52f4e259f5f95c83c71e72f065093da.1616966903.git.lucas.p.stankus@gmail.com


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