net: stmmac: Add support for HW-accelerated VLAN stripping

Current implementation supports driver level VLAN tag stripping only.
The features is always on if CONFIG_VLAN_8021Q is enabled in kernel
config and is not user configurable.

This patch add support to MAC level VLAN tag stripping and can be
configured through ethtool. If the rx-vlan-offload is off, the VLAN tag
will be stripped by driver. If the rx-vlan-offload is on, the VLAN tag
will be stripped by MAC.

Command: ethtool -K <interface> rx-vlan-offload off | on

Signed-off-by: Lai Peter Jun Ann <jun.ann.lai@intel.com>
Signed-off-by: Gan, Yi Fang <yi.fang.gan@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>