gpio: pca953x: Extract the register address mangling to single function

Instead of having the I2C register calculation function spread across
multiple accessor functions, pull it out into a single function which
returns the adjusted register address.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>