xhci: Always use usb_hcd in URB instead of converting xhci_hcd.

Make sure to call into the USB core's link, unlink, and giveback URB
functions with the usb_hcd pointer found by using urb->dev->bus.  This
will avoid confusion later, when the xHCI driver will deal with URBs from
two separate buses (the USB 3.0 roothub and the faked USB 2.0 roothub).

Assume xhci_urb_dequeue() will be called with the proper usb_hcd.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>