net/mlx5: Simplify a test

'create_root_ns()' does not return an error pointer, so the test can be
simplified to be more consistent.
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

@@ -1690,7 +1690,7 @@ static int init_root_ns(struct mlx5_flow_steering *steering)
 {
 
 	steering->root_ns = create_root_ns(steering, FS_FT_NIC_RX);
-	if (IS_ERR_OR_NULL(steering->root_ns))
+	if (!steering->root_ns)
 		goto cleanup;
 
 	if (init_root_tree(steering, &root_fs, &steering->root_ns->ns.node))