usb: mtu3: clear SOFTCONN when clear USB3_EN if work as HS mode

When the controller supports SS mode, but works as HS mode, the
SOFTCONN will not be cleared automatically when clear USB3_EN
by default, this cause an issue that can't disconnect from host,
so clear SOFTCONN when clear USB3_EN when the class driver want
to disable the D+ pullup.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>