net/mlx5: Fix mlx5_add_flow_rules call with correct num of dests

When adding ethtool steering rule with action DISCARD we wrongly
pass a NULL dest with dest_num 1 to mlx5_add_flow_rules().
What this error seems to have caused is sending VPORT 0
(MLX5_FLOW_DESTINATION_TYPE_VPORT) as the fte dest instead of no dests.
We have fte action correctly set to DROP so it might been ignored
anyways.

To reproduce use:
 # sudo ethtool --config-nfc <dev> flow-type ether \
   dst aa:bb:cc:dd:ee:ff action -1

Fixes: 74491de9 ("net/mlx5: Add multi dest support")
Signed-off-by: Paul Blakey <paulb@mellanox.com>
Reviewed-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>

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

@@ -320,7 +320,7 @@ add_ethtool_flow_rule(struct mlx5e_priv *priv,
 
 	spec->match_criteria_enable = (!outer_header_zero(spec->match_criteria));
 	flow_act.flow_tag = MLX5_FS_DEFAULT_FLOW_TAG;
-	rule = mlx5_add_flow_rules(ft, spec, &flow_act, dst, 1);
+	rule = mlx5_add_flow_rules(ft, spec, &flow_act, dst, dst ? 1 : 0);
 	if (IS_ERR(rule)) {
 		err = PTR_ERR(rule);
 		netdev_err(priv->netdev, "%s: failed to add ethtool steering rule: %d\n",