nfp: support UDP segmentation offload

The device supports UDP hardware segmentation offload, which helps
improving the performance. Thus, this patch adds support for UDP
segmentation offload from the driver side.

Signed-off-by: Fei Qin <fei.qin@corigine.com>
Signed-off-by: Louis Peens <louis.peens@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>