Instead of allocating space for the whole xhci_hcd structure at the end of
usb_hcd, make the USB core allocate enough space for a pointer to the
xhci_hcd structure.  This will make it easy to share the xhci_hcd
structure across the two roothubs (the USB 3.0 usb_hcd and the USB 2.0
usb_hcd).

Deallocate the xhci_hcd at PCI remove time, so the hcd_priv will be
deallocated after the usb_hcd is deallocated.  We do this by registering a
different PCI remove function that calls the usb_hcd_pci_remove()
function, and then frees the xhci_hcd.  usb_hcd_pci_remove() calls
kput() on the usb_hcd structure, which will deallocate the memory that
contains the hcd_priv pointer, but not the memory it points to.
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>