gpio: pca953x: Unify pca953x_{read,write}_regs_{16,24}()

At this point, these two functions only differ in whether they do or do not
set the address increment bit. The 16 GPIO case does not need to set the AI
bit, except for PCA9575 on write, while the 24 GPIO and more case does set
the AI bit always. Merge these two functions together to simplify the code
a bit.

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>