ip-tunnel: set cloned-mac-address only for layer2 tunnel devices

For ip-tunnel modes that encapsulate layer2 packets (gretap and ip6gretap) we allow the presence of an ethernet setting in the connection and honor the cloned-mac-address specified in it.

For all other modes, the ethernet setting is removed during normalization, but a value different from 'preserve' could be set via global default.

The kernel doesn't allow setting a MAC for layer3 devices, don't do it.