cxgb4vf: do vlan cleanup

- unify vlan and nonvlan rx path
- kill pi->vlan_grp and cxgb4vf_vlan_rx_register
- allow to turn on/off rx/tx vlan accel via ethtool (set_features)

Signed-off-by: Jiri Pirko <jpirko@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>