gpio: zynq: Reduce level of indention in zynq_gpio_irqhandler()

zynq_gpio_irqhandler() uses up to 7 tabs of indention in some parts. Refactor
things to use a helper function for the inner loop to reduce the indention to a
sane level.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>