An EUI provider can provide EUIs for multiple interfaces based on
their index.
For this is should get the index of the interface, not the index of
the EUI provider.
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.
