nfp: support LSO2 capability

Firmware advertising the LSO2 capability exploits driver provided L3 and L4
offsets in order to avoid parsing packet headers in the TX path. The vlan
field in struct nfp_net_tx_desc is repurposed, making TXVLAN a mutually
exclusive configuration to LSO2.

Signed-off-by: Edwin Peer <edwin.peer@netronome.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>