TTY: isdn/gigaset destroy tty_port properly

Currently, the port may be destroyed twice or destroyed without being
initialized. Fix this by sticking the destroy call in the right place
as suggested by Tilman.

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