stmmac: Add vlan rx for better GRO performance.

GRO requires VLANs to be removed before aggregation can occur.
The Synopsys EMAC does not strip VLAN tags so this must be
done by the driver.

Signed-off-by: Vince Bridgers <vbridgers2013@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>