selftests: forwarding: devlink_lib: Support port-less topologies

Some selftests may not need any actual ports. Technically those are not
forwarding selftests, but devlink_lib can still be handy. Fall back on
NETIF_NO_CABLE in those cases.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>