USB: usb-serial: fix a sparse warning about different signedness

fix the following sparse warning:

drivers/usb/serial/usb-serial.c:927:43: warning: incorrect type in argument 3 (different signedness)
drivers/usb/serial/usb-serial.c:927:43:    expected unsigned int *minor
drivers/usb/serial/usb-serial.c:927:43:    got int *<noident>
  CHECK   drivers/usb/serial/generic.c
Signed-off-by: Andre Haupt <andre@bitwigglers.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/serial/usb-serial.c

@@ -625,7 +625,7 @@ int usb_serial_probe(struct usb_interface *interface,
 	struct usb_endpoint_descriptor *bulk_out_endpoint[MAX_NUM_PORTS];
 	struct usb_serial_driver *type = NULL;
 	int retval;
-	int minor;
+	unsigned int minor;
 	int buffer_size;
 	int i;
 	int num_interrupt_in = 0;