TTY: use tty_wait_until_sent_from_close in tty_port_close_start

Let's use the newly added helper to avoid stalls in drivers which are
already ported to tty_port helpers.

We have to ensure here, that there is no user of tty_port_close_start
and tty_port_close which holds port->mutex (or other) lock over them.
And sure, there is none.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>