gpio: fix meaningless return expression

Fix the following sparse error:

drivers/gpio/gpio-ath79.c:54:16: error: return expression in void function

Signed-off-by: zhong jiang <zhongjiang@huawei.com>
Acked-by: Alban Bedel <albeu@free.fr>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>