Trivial: xhci: Fix copy-paste error.

The xHCI driver will create an xhci_hcd structure, not an ehci_hci
structure.
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>

drivers/usb/host/xhci.h

@@ -1356,7 +1356,7 @@ static inline unsigned int hcd_index(struct usb_hcd *hcd)
 		return 1;
 }
 
-/* There is one ehci_hci structure per controller */
+/* There is one xhci_hcd structure per controller */
 struct xhci_hcd {
 	struct usb_hcd *main_hcd;
 	struct usb_hcd *shared_hcd;