discover/network: Ensure interfaces have device before configuring

Reorganise network_handle_nlmsg() slightly to create interface->dev just
before calling configure_interface() rather than only for brand new
interfaces. This ensures existing interfaces which have had ->dev
removed but receive a new configure event do not access a NULL pointer
during the configuration process.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>