nfp: support RX VLAN ctag/stag strip

Add support for RX VLAN ctag/stag strip
which may be configured via ethtool.

e.g.
     # ethtool -K $DEV rx-vlan-offload on
     # ethtool -K $DEV rx-vlan-stag-hw-parse on

Ctag-stripped and stag-stripped cannot be enabled at the same time
because currently the kernel supports only one layer of VLAN stripping.

The NIC supplies VLAN strip information as packet metadata.
The fields of this VLAN metadata are:

* strip flag: 1 for stripped; 0 for unstripped
* tci: VLAN TCI ID
* tpid: 1 for ETH_P_8021AD; 0 for ETH_P_8021Q

Configuration control bits NFP_NET_CFG_CTRL_RXVLAN_V2 and
NFP_NET_CFG_CTRL_RXQINQ are to signal availability of
ctag-strip and stag-strip features of the firmware.

Signed-off-by: Diana Wang <na.wang@corigine.com>
Reviewed-by: Louis Peens <louis.peens@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>