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: Wei Yongjun <weiyj.lk@gmail.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>