phy: mediatek: Mark mtk_mipi_tx_driver with static keyword

Fix the following sparse warning:

drivers/phy/mediatek/phy-mtk-mipi-dsi.c:237:24: warning: symbol 'mtk_mipi_tx_driver' was not declared. Should it be static?

Signed-off-by: Zou Wei <zou_wei@huawei.com>
Link: https://lore.kernel.org/r/1610415484-92497-1-git-send-email-zou_wei@huawei.com


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