iio: adc: ad7923: use devm_add_action_or_reset for regulator disable

Adds a device-managed action to handle disabling the driver's regulator on
device detach.
This slightly simplifies deinitialization and enables further conversion of
the driver to device-managed routines without breaking the init order.

Signed-off-by: Lucas Stankus <lucas.p.stankus@gmail.com>
Reviewed-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Link: https://lore.kernel.org/r/49a7c0436ca1186313dbccf3d810d0cf38cb5b37.1616966903.git.lucas.p.stankus@gmail.com


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