gpio: pca953x: refactor pca953x_write_regs()

Avoid the unnecessary if-else in pca953x_write_regs() by splitting
the routine into smaller, specialized functions and calling the right
one via a function pointer held in struct pca953x_chip.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>