xhci: Add a SuperSpeedPlus capability descriptor for xhci USB 3.1 roothub

All usb devices that support USB 3.1 Gen2 speeds need to provide a
SuperSpeedPlus device capability descriptor as part of their BOS
descriptor.

If the xhci controller supports USB 3.1 enhanced SuperSpeed, meaning
it can handle both Gen1 SuperSpeed 5Gbps and Gen2 SuperSpeedPlus 10Gbps
devices, then we need to provide a SuperSpeedPlus capability descriptor
for the USB 3.1 roothub as well.

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