tty: tty_port_open, document shutdown vs failed activate

Add a note that ->shutdown is not called when ->activate fails. Just so
we are clear.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220124071430.14907-5-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>