usb: xhci: fix build warning - missing prototype

Fix build warning when building drivers/usb/host/xhci-mem.o with W=1 due
to missing prototype for xhci_free_virt_devices_depth_first.

This function is only used in xhci-mem.c so just make it static.
Signed-off-by: Jean-Philippe Menil <jpmenil@gmail.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/host/xhci-mem.c

@@ -933,7 +933,7 @@ void xhci_free_virt_device(struct xhci_hcd *xhci, int slot_id)
  * that tt_info, then free the child first. Recursive.
  * We can't rely on udev at this point to find child-parent relationships.
  */
-void xhci_free_virt_devices_depth_first(struct xhci_hcd *xhci, int slot_id)
+static void xhci_free_virt_devices_depth_first(struct xhci_hcd *xhci, int slot_id)
 {
 	struct xhci_virt_device *vdev;
 	struct list_head *tt_list_head;