xhci: define the new default speed ID for SuperSpeedPlus used by xhci hw

USB 3.1 capable xhci controllers use a new default speed ID "5" in the
PORTSC register to represent a 10Gbps connection speed of a SuperSpeedPlus
device

Make sure the xhci driver can handle the returned SuperSpeedPlus speed ID
properly

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