staging: iio: ade7854: use devm_iio_device_register

Replace iio_device_register with the device managed version.
This change is safe because it does not change the order of
any device removal actions. Unregistering the device was the
only removal action. The newly emptied .remove functions are
deleted.

Signed-off-by: Alison Schofield <amsfield22@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>