staging: dgap: introduce dgap_tty_free() for freeing channels.

It should be called after dgap_tty_register_ports() is failed.
So channels which are allocated in dgap_tty_init() will be freed.

Signed-off-by: Daeseok Youn <daeseok.youn@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>