gpio: pca953x: Unify pca95{3,7}x_write_regs_16()

At this point, these two functions only differ in whether they do or do not
set the address increment bit on PCA9575. 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>