cxgb4/cxgb4vf: add support for ndo_set_vf_vlan

implement ndo_set_vf_vlan for mgmt netdevice to configure
the PCIe VF.

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