MIPS: ath79: simplify ath79_gpio_function_* routines

Make ath79_gpio_function_{en,dis}able to be wrappers
around ath79_gpio_function_setup.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/4871/


Signed-off-by: John Crispin <blogic@openwrt.org>