n_tty: update comment for WAKEUP_CHARS define

Give a better descriptions of what WAKEUP_CHARS represents.

Signed-off-by: Valentin Vidic <Valentin.Vidic@CARNet.hr>
Acked-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>