staging:iio:dac:ad5791: allocate chip state with iio_dev and use iio_priv for access.

Slightly fiddly case with two regulators - could reorder, but this is the approach with
smallest likely impact.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>