gianfar: keep vlan related state when restart

If vlan has been enabled. ifdown followed by ifup will lost hardware
related state.

Also remove duplicated operation in gfar_vlan_rx_register().

Signed-off-by: Yong Zhang <yong.zhang0@gmail.com>
Acked-by: Dai Haruki <dai.haruki@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>