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>

drivers/pinctrl/mediatek/pinctrl-mtk-common-v2.c

@@ -34,7 +34,7 @@ struct mtk_drive_desc {
 };
 
 /* The groups of drive strength */
-const struct mtk_drive_desc mtk_drive[] = {
+static const struct mtk_drive_desc mtk_drive[] = {
 	[DRV_GRP0] = { 4, 16, 4, 1 },
 	[DRV_GRP1] = { 4, 16, 4, 2 },
 	[DRV_GRP2] = { 2, 8, 2, 1 },