cxgb4: implement ndo_set_vf_rate()

Implement ndo_set_vf_rate() for mgmt interface to support rate-limiting
of VF traffic using 'ip' command.

Based on the original work of Kumar Sanghvi <kumaras@chelsio.com>

Signed-off-by: Ganesh Goudar <ganeshgr@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>