TTY: msm_smd_tty, clean up activate/shutdown

Do not dig struct smd_tty_info out of tty_struct using
tty_port_tty_get. It is unnecessarily too complicated, use simple
container_of instead.

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