MIPS: ath79: Change number of available IRQs

The status register of the miscellaneous interrupt controller is 32 bits
wide, but the actual value of NR_IRQS covers only 8 of them. Change
NR_IRQS in order to make all of those interrupt lines usable.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2441/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>