xhci: set hcd pointers for xhci usb2 and usb3 roothub structures

Allows us to know the correct hcd a xhci roothub and its ports
belong to.

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