staging: iio: cdc: ad7746: extract capac setup to own function

Refactor the capdac register write logic to own function.

Also fixes the following checkpatch warning:
CHECK: Alignment should match open parenthesis

Signed-off-by: Lucas Stankus <lucas.p.stankus@gmail.com>
Reviewed-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Link: https://lore.kernel.org/r/b637a5ecde4e5f9f927a5b104332378d4721c91d.1621786036.git.lucas.p.stankus@gmail.com


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