ttyprintk: Fix wrong tty_unregister_driver() call in the error path

ttyprintk driver calls tty_unregister_driver() wrongly in the error
path of tty_register_driver().  Also, setting ttyprintk_driver to NULL
is utterly superfluous, so let's get rid of it, too.
Reported-by: Jean Delvare <jdelvare@suse.de>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/char/ttyprintk.c

@@ -210,10 +210,8 @@ static int __init ttyprintk_init(void)
 	return 0;
 
 error:
-	tty_unregister_driver(ttyprintk_driver);
 	put_tty_driver(ttyprintk_driver);
 	tty_port_destroy(&tpk_port.port);
-	ttyprintk_driver = NULL;
 	return ret;
 }
 device_initcall(ttyprintk_init);