rtnetlink: allow to register ops without ops->setup set

So far, it is assumed that ops->setup is filled up. But there might be
case that ops might make sense even without ->setup. In that case,
forbid to newlink and dellink.

This allows to register simple rtnl link ops containing only ->kind.
That allows consistent way of passing device kind (either device-kind or
slave-kind) to userspace.

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>