rocker: use new helper to figure out master kind

Looking at rtnl kind string is kind of ugly. So use new helpers to do
this in nicer way.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Acked-by: Scott Feldman <sfeldma@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>