batman-adv: Act on NETDEV_*_TYPE_CHANGE events

A network interface can change type. It may change from a type which
batman does not support, e.g. hdlc, to one it does, e.g. hdlc-eth.
When an interface changes type, it sends two notifications. Handle
these notifications.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>