iio: adc: stx104: Implement and utilize register structures

Reduce magic numbers and improve code readability by implementing and
utilizing named register data structures.

Tested-by: Fred Eckert <Frede@cmslaser.com>
Signed-off-by: William Breathitt Gray <william.gray@linaro.org>
Link: https://lore.kernel.org/r/8cb91d5b53e57b066120e42ea07000d6c7ef5543.1657213745.git.william.gray@linaro.org


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