staging:iio:various cleanup regulator use paths.

No way iio_device_allocate or iio_device_unregister effected
by regulators so copying tricks not needed.

This is really about regularizing the layout of drivers.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>