iio: adc: axp20x_adc: Minor code cleanups

iio: adc: axp20x_adc: Minor code cleanups

The code may be clearer if parameters are not re-purposed to hold
temporary results like register values, so introduce local variables
as necessary to avoid that. Regroup macros based on chip type, and
use the FIELD_PREP() macro instead of a hand-rolled version.

Suggested-by: Jonathan Cameron <jic23@kernel.org>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Aidan MacDonald <aidanmacdonald.0x0@gmail.com>
Link: https://lore.kernel.org/r/20230217153404.32481-1-aidanmacdonald.0x0@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>