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>