net/mlx5e: Correctly report errors for ethtool rx flows

Previously, an ethtool rx flow with no attrs would not be added to the
NIC as it has no rules to configure the hw with, but it would be
reported as successful to the caller (return code 0). This is confusing
for the user as ethtool then reports "Added rule $num", but no rule was
actually added.

This change corrects that by instead reporting these wrong rules as
-EINVAL.

Fixes: b29c61dac3 ("net/mlx5e: Ethtool steering flow validation refactoring")
Signed-off-by: Cosmin Ratiu <cratiu@nvidia.com>
Reviewed-by: Saeed Mahameed <saeedm@nvidia.com>
Reviewed-by: Dragos Tatulea <dtatulea@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://patch.msgid.link/20240808144107.2095424-5-tariqt@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/mellanox/mlx5/core/en_fs_ethtool.c

@@ -734,7 +734,7 @@ mlx5e_ethtool_flow_replace(struct mlx5e_priv *priv,
 	if (num_tuples <= 0) {
 		netdev_warn(priv->netdev, "%s: flow is not valid %d\n",
 			    __func__, num_tuples);
-		return num_tuples;
+		return num_tuples < 0 ? num_tuples : -EINVAL;
 	}
 
 	eth_ft = get_flow_table(priv, fs, num_tuples);