staging: dgap: unwind on error in dgap_tty_register_ports()

- The dgap_tty_register_ports() needs to handle if the
tty_port_register_device() fails.

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