gpio: fix aspeed_gpio unmask irq

commit f241632fd087d3d9fbd5450f4d8c8604badd8348 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>