ttyprintk: Allow built as a module

The driver is well written to be used as a module, just the exit call
is missing.

Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>