iio: adc: ad7124: Make register naming consistent

iio: adc: ad7124: Make register naming consistent

Cleanup definition of register related constants:

 - Use the register and field names exactly as documented in the data
   sheet.
 - Consistently use <regname>_<bitfield> to name a register's bitfield.
 - Drop _MSK definitions and implicit FIELD_PREP calls.
 - Consistent indentation.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://patch.msgid.link/20250317115247.3735016-8-u.kleine-koenig@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>