pinctrl: mediatek: make symbol 'mtk_drive' static

Fixes the following sparse warning:

drivers/pinctrl/mediatek/pinctrl-mtk-common-v2.c:37:29: warning:
 symbol 'mtk_drive' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>