TTY: isdn/gigaset, use tty_port

Let us port the code to use tty_port. We now use open_count and tty
from there. This allows us also to use tty_port_tty_set with tty
refcounting instead of hand-written locking and logic.

Note that tty and open_count are no longer protected by cs->lock. It is
protected by tty_port->lock. But since all the places where they were
used are now switched to the helpers, we are fine.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Hansjoerg Lipp <hjlipp@web.de>
Acked-by: Tilman Schmidt <tilman@imap.cc>
Cc: <gigaset307x-common@lists.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>