usb: xhci-mtk: supports SSP without external USB3 gen2 hub

Supports SSP scheduling only for SSP device directly connected
to root hub but not through external USB3 gen2 hub which need
use a new scheduling way.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>