usb-serial: fix usb_serial_register bug when boot with nousb param

With "nousb" cmdline booting, built-in serial drivers (ie. airecable)
will trigger kernel oops.

Indeed, if nousb, usb_serial_init will failed, and the usb serial bus type
will not be registerd, then usb_serial_register call driver_register
which try to register the driver to a not registered bus.

Here add usb_disabled() check in usb_serial_register to fix it.
Signed-off-by: Dave Young <hidave.darkstar@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/serial/usb-serial.c

@@ -1241,6 +1241,9 @@ int usb_serial_register(struct usb_serial_driver *driver)
 	/* must be called with BKL held */
 	int retval;
 
+	if (usb_disabled())
+		return -ENODEV;
+
 	fixup_generic(driver);
 
 	if (!driver->description)