pinctrl: sunxi: Read register before writing to it in irq_set_type

The current irq_set_type code doesn't read the current register value
before writing to it, leading to the older programmed values being
overwritten and everything but the latest value being reset.

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