net/mlx5: Fix 'DON'T_TRAP' functionality

The flow counters binding support commit introduced a code change where
none NULL 'rule_dest' is always passed to mlx5_add_flow_rules, this breaks
'DON'T_TRAP' rules insertion.

The fix uses the equivalent 'dest_num' value instead of dest pointer
at the failed check.

fixes: 3b3233f ('IB/mlx5: Add flow counters binding support')
Signed-off-by: Raed Salem <raeds@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>

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

@@ -1887,7 +1887,7 @@ mlx5_add_flow_rules(struct mlx5_flow_table *ft,
 	if (flow_act->action == MLX5_FLOW_CONTEXT_ACTION_FWD_NEXT_PRIO) {
 		if (!fwd_next_prio_supported(ft))
 			return ERR_PTR(-EOPNOTSUPP);
-		if (dest)
+		if (dest_num)
 			return ERR_PTR(-EINVAL);
 		mutex_lock(&root->chain_lock);
 		next_ft = find_next_chained_ft(prio);