xhci: fix non static symbol warning

Fixes the following sparse warning:

drivers/usb/host/xhci-ring.c:608:6: warning:
 symbol 'xhci_unmap_td_bounce_buffer' was not declared.
 Should it be static?
Signed-off-by: NWei Yongjun <weiyj.lk@gmail.com>
Signed-off-by: NMathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/host/xhci-ring.c

@@ -586,8 +586,8 @@ static void xhci_giveback_urb_in_irq(struct xhci_hcd *xhci,
 	}
 }
 
-void xhci_unmap_td_bounce_buffer(struct xhci_hcd *xhci, struct xhci_ring *ring,
-				 struct xhci_td *td)
+static void xhci_unmap_td_bounce_buffer(struct xhci_hcd *xhci,
+		struct xhci_ring *ring, struct xhci_td *td)
 {
 	struct device *dev = xhci_to_hcd(xhci)->self.controller;
 	struct xhci_segment *seg = td->bounce_seg;