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 <kexin.hao@windriver.com>
Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/usb/serial/console.c

@@ -135,6 +135,7 @@ static int usb_console_setup(struct console *co, char *options)
 				err("no more memory");
 				goto reset_open_count;
 			}
+			kref_init(&tty->kref);
 			termios = kzalloc(sizeof(*termios), GFP_KERNEL);
 			if (!termios) {
 				retval = -ENOMEM;