staging: dgnc: introduce the dgnc_tty_unregister()

The dgnc_tty_unregister() will be called when
the dgnc_tty_register() is failed.

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