nfp: flower-ct: check for error in nfp_fl_ct_offload_nft_flow()

The nfp_fl_ct_add_flow() function can fail so we need to check for
failure.

Fixes: 95255017

 ("nfp: flower-ct: add nft flows to nft list")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Louis Peens <louis.peens@corigine.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>