staging:iio:addac:adt7316: allocate chip state with iio_dev and use iio_priv to access.

Various minor cleanups needed to deal with removal of iio_dev pointer from chip state.
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>