gpio: fix aspeed_gpio unmask irq

The unmask function disables all interrupts in a bank when unmasking an
interrupt. Only disable the given interrupt.

Cc: stable@vger.kernel.org
Signed-off-by: Govert Overgaauw <govert.overgaauw@prodrive-technologies.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>