gpio: gpio-aspeed-sgpio: Fix wrong hwirq in irq handler.

The current hwirq is calculated based on the old GPIO pin order(input
GPIO range is from 0 to ngpios - 1).
It should be calculated based on the current GPIO input pin order(input
GPIOs are 0, 2, 4, ..., (ngpios - 1) * 2).

Signed-off-by: Steven Lee <steven_lee@aspeedtech.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>