usbip: rate limit get_frame_number message

It's annoying to constantly see the same "Not yet implemented" message
over and over with nothing able to be done about it, so rate limit it
for now to keep user's logs "clean".
Reported-by: NLars Täuber <lars.taeuber@web.de>
Tested-by: NLars Täuber <lars.taeuber@web.de>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/usbip/vhci_hcd.c

@@ -941,7 +941,7 @@ static void vhci_stop(struct usb_hcd *hcd)
 
 static int vhci_get_frame_number(struct usb_hcd *hcd)
 {
-	pr_err("Not yet implemented\n");
+	dev_err_ratelimited(&hcd->self.root_hub->dev, "Not yet implemented\n");
 	return 0;
 }