TTY: ircomm, define local tty_port

In some functions we use tty_port heavily. So let us have a local
pointer to that variable instead of having self->port all over the
code.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Samuel Ortiz <samuel@sortiz.org>
Cc: netdev@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>