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

drivers/phy/mediatek/phy-mtk-mipi-dsi.c

@@ -234,7 +234,7 @@ static const struct of_device_id mtk_mipi_tx_match[] = {
 	{ },
 };
 
-struct platform_driver mtk_mipi_tx_driver = {
+static struct platform_driver mtk_mipi_tx_driver = {
 	.probe = mtk_mipi_tx_probe,
 	.remove = mtk_mipi_tx_remove,
 	.driver = {