gpio: uniphier: Fix void functions to remove return value

The return type of irq_chip.irq_mask() and irq_chip.irq_unmask() should
be void.

Fixes: dbe776c2

 ("gpio: uniphier: add UniPhier GPIO controller driver")
Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>