soc: mediatek: Make symbol 'mtk_mutex_driver' static

The sparse tool complains as follows:

drivers/soc/mediatek/mtk-mutex.c:464:24: warning:
 symbol 'mtk_mutex_driver' was not declared. Should it be static?

This symbol is not used outside of mtk-mutex.c, so this
commit marks it static.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Link: https://lore.kernel.org/r/20210210075656.1096251-1-weiyongjun1@huawei.com


Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>