TTY: isdn, define local tty_port

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

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Karsten Keil <isdn@linux-pingi.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>