TTY: hso, add tty_port

And use open count from there. Other members will follow.

Remark: port.count is (and never was) properly protected. Only a mutex
is held, so ISR and all the functions it calls may see an invalid
state.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Jan Dumon <j.dumon@option.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>