Add kref to fake tty used by USB console

We alloc a fake tty in usb serial console setup function. we should
init the tty's kref otherwise we will face WARN_ON after following
invoke of tty_port_tty_set --> tty_kref_get.

Signed-off-by: Kevin Hao <[e-mail]>
Signed-off-by: Alan Cox <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/usb/serial/console.c

···
 				err("no more memory");
 				goto reset_open_count;
 			}
+			kref_init(&tty->kref);
 			termios = kzalloc(sizeof(*termios), GFP_KERNEL);
 			if (!termios) {
 				retval = -ENOMEM;