net: stmmac: enable HW-accelerated VLAN stripping for gmac4 only

Commit 750011e2 ("net: stmmac: Add support for HW-accelerated VLAN
stripping") enables MAC level VLAN tag stripping for all MAC cores, but
leaves set_hw_vlan_mode() and rx_hw_vlan() un-implemented for both gmac
and xgmac.

On gmac and xgmac, ethtool reports rx-vlan-offload is on, both MAC and
driver do nothing about VLAN packets actually, although VLAN works well.

Driver level stripping should be used on gmac and xgmac for now.

Fixes: 750011e2

 ("net: stmmac: Add support for HW-accelerated VLAN stripping")
Signed-off-by: Furong Xu <0x1207@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>