nfp: fix esp-tx-csum-offload doesn't take effect

When esp-tx-csum-offload is set to on, the protocol stack shouldn't
calculate the IPsec offload packet's csum, but it does. Because the
callback `.ndo_features_check` incorrectly masked NETIF_F_CSUM_MASK bit.

Fixes: 57f273ad

 ("nfp: add framework to support ipsec offloading")
Signed-off-by: Huanhuan Wang <huanhuan.wang@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>