phy: phy-mt65xx-usb3: improve RX detection stable time

The default value of RX detection stable time is 10us, and this
margin is too big for some critical cases which cause U3 link fail
and link to U2(probability is about 1%). So change it to 5us.

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