xhci: check xhci hardware for USB 3.1 support

Set the controller speed to HCD_USB31 to if host hardware supports USB 3.1

For PCI xhci controllers the USB 3.1 support is checked from SBRN bits in
pci config space. Platform controllers will need to set xhci->sbrn == 0x31
to indicate USB 3.1 support before calling xhci_gen_setup().

Also make sure xhci driver works correctly with speed set to HCD_USB31

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