USB: fix __must_check warnings in drivers/usb/serial/

Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/drivers/usb/serial/usb-serial.c b/drivers/usb/serial/usb-serial.c
index 1f21a9d..03c6194 100644
--- a/drivers/usb/serial/usb-serial.c
+++ b/drivers/usb/serial/usb-serial.c
@@ -932,7 +932,10 @@
 
 		snprintf (&port->dev.bus_id[0], sizeof(port->dev.bus_id), "ttyUSB%d", port->number);
 		dbg ("%s - registering %s", __FUNCTION__, port->dev.bus_id);
-		device_register (&port->dev);
+		retval = device_register(&port->dev);
+		if (retval)
+			dev_err(&port->dev, "Error registering port device, "
+				"continuing\n");
 	}
 
 	usb_serial_console_init (debug, minor);