phy: phy-mtk-tphy: keep default value of mcu_bus_ck_gate_en

The default value of mcu_bus_ck_gate_en is 1, if clear it, will
prevent system to enter deep idle mode, so keep its default value
and without affecting PCIe function.

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