usb: mtu3: enable auto switch from U3 to U2

inform mac2 to build U2 link automatically after U3 detect
fail without software setting soft_connect.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>