staging: iio: cdc: ad7746: add additional config defines

Introduce defines for shifting and mask under the config register for
better readability. Also, introduce helper variables for index
calculation.

Signed-off-by: Eva Rachel Retuya <eraretuya@gmail.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>