MIPS: AU1000: Fix build failure for db1x00 configured for Au1100 SoC

This patch fixes the following warning, which becomes an error due to
-Werror to be turned on:
  CC      arch/mips/alchemy/common/gpiolib-au1000.o
cc1: warnings being treated as errors
arch/mips/alchemy/common/gpiolib-au1000.c: In function 'au1100_gpio2_to_irq':
/home/florian/dev/kernel/linux-queue/arch/mips/include/asm/mach-au1x00/gpio-au1000.h:107: warning: control reaches end of non-void function

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