gpio: ws16c48: Implement and utilize register structures

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

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Cc: Paul Demetrotion <pdemetrotion@winsystems.com>
Signed-off-by: William Breathitt Gray <william.gray@linaro.org>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>