Staging: IIO: DAC: AD5624R: Consistency cleanup - no functional changes

Consistently use indio_dev and access macro for devdata

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