phy: phy-mtk-tphy: use auto instead of force to bypass utmi signals

When system is running, if usb2 phy is forced to bypass utmi signals,
all PLL will be turned off, and it can't detect device connection
anymore, so replace force mode with auto mode which can bypass utmi
signals automatically if no device attached for normal flow.
But keep the force mode to fix RX sensitivity degradation issue.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>