nfp: rename l4_offset in struct nfp_net_tx_desc to lso_hdrlen

The l4_offset field referred to by NFD is confusingly named. It is not the
offset of the L4 transport header, but rather the L4 payload.

The LSO2 capability supported by alternative device firmware requires
the actual L4 offset, thus the rename seems prudent.

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