staging:iio:dac:ad5791: Use correct DAC bit-size

Commit c5b99396 ("staging:iio:dac:ad5791 chan spec conversion.") introduced a
small bug, using storagebits instead of realbits throughout the driver, which
causes the driver to work incorrectly. This patch fixes it.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>