iio: adc: ad7949: define and use bitfield names

Replace raw configuration register values by using FIELD_PREP and
defines to improve readability.

Signed-off-by: Liam Beguin <lvb@xiphos.com>
Link: https://lore.kernel.org/r/20210815213309.2847711-2-liambeguin@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>