The EUI provider function only gets the index of a device within it's
device type.
Using NETDEV_ANY with two devices of different type causes the EUI
provider to be used for both (since both interfaces are index 0 of
their type).
To prevent this, require EUI providers to be locked to an interface type.
If the same callback function is used for multiple interfaces
(`NETDEV_INDEX_ANY`), it is necessary to also provide the index of
the interface to hand out and address.
