TTY: n_gsm, use tty_port_install

We need to link a port to a tty in install. And since dlci is
allocated even in open, we need to create gsmtty_install, allocate
dlci there and create also the link.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>