phy: phy-mtk-dp: change mtk_dp_phy_driver to static

mtk_dp_phy_driver is only used in phy-mtk-dp.c now, change it to static.

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Link: https://lore.kernel.org/r/20220707135309.801181-1-yangyingliang@huawei.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>