pinctrl: sunxi: Remove irq_mask_ack and use irq_ack instead

If irq_mask_ack is not defined, mask_ack_irq will call irq_mask and then
irq_ack. In order to avoid code duplication, between irq_mask_ack and irq_mask,
just declare irq_ack.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>