IB/mlx4: Don't register IB device for adapters with no IB ports

If the mlx4_ib driver finds an adapter that has only ethernet ports, the
current code will register an IB device with 0 ports.  Nothing useful or
sensible can be done with such a device, so just skip registering it.

Signed-off-by: Roland Dreier <rolandd@cisco.com>