MIPS: RB532: Check irq number when handling GPIO interrupts

This patch makes sure that we are not going to clear
or change the interrupt status of a GPIO interrupt
superior to 13 as this is the maximum number of GPIO
interrupt source (p.232 of the RC32434 reference manual).

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>