iio: adc: stx104: Implement and utilize register structures

iio: adc: stx104: Implement and utilize register structures

[ Upstream commit 6cfd14c54b1f42f29097244c1b6208f8268d7d5b ]

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>
Stable-dep-of: 4f9b80aefb9e ("iio: addac: stx104: Fix race condition when converting analog-to-digital")
Signed-off-by: Sasha Levin <sashal@kernel.org>