gpio: pca953x: Zap ad-hoc I2C block write in multi GPIO set

The ad-hoc i2c block write can be replaced by standard register accessor
function, which correctly handles all the chip details and differences.
Do so to simplify the code.

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>