gianfar: do vlan cleanup

- unify vlan and nonvlan rx path
- kill priv->vlgrp and gfar_vlan_rx_register
- allow to turn on/off rx/tx vlan accel via ethtool

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