net: cdc-phonet: fix data-interface release on probe failure

Set the disconnected flag before releasing the data interface in case
netdev registration fails to avoid having the disconnect callback try to
deregister the never registered netdev (and trigger a WARN_ON()).

Fixes: 87cf6560

 ("USB host CDC Phonet network interface driver")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>