usb: xhci: make function xhci_dbc_free_req static

Function xhci_dbc_free_req is local to the source and does not need to
be in global scope, so make it static.

Cleans up sparse warning:
symbol 'xhci_dbc_free_req' was not declared. Should it be static?
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Cc: Mathias Nyman <mathias.nyman@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/host/xhci-dbgtty.c

@@ -122,7 +122,7 @@ static void dbc_write_complete(struct xhci_hcd *xhci, struct dbc_request *req)
 	spin_unlock(&port->port_lock);
 }
 
-void xhci_dbc_free_req(struct dbc_ep *dep, struct dbc_request *req)
+static void xhci_dbc_free_req(struct dbc_ep *dep, struct dbc_request *req)
 {
 	kfree(req->buf);
 	dbc_free_request(dep, req);