gpiolib: fix OOB access in quirk callbacks

Commit a2b5e207 ("gpiolib: rework quirk handling in of_find_gpio()")
introduced an array of quirk functions which get iterated over. But a
sentinal value is missing. Add it.

Fixes: a2b5e207

 ("gpiolib: rework quirk handling in of_find_gpio()")
Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Tested-by: Biju Das <biju.das.jz@bp.renesas.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>