gpio: of: move chip->of_gpio_n_cells checking to of_gpiochip_add()

Do this sanity check only once when the gpio_chip is added
rather than every time gpio-hog is handled.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>