net/mlx5: Use ERR_CAST() instead of coding it

This makes it more readable that rule is being used to return an err.

Signed-off-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>

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

@@ -1825,7 +1825,7 @@ search_again_locked:
 
 	g = alloc_auto_flow_group(ft, spec);
 	if (IS_ERR(g)) {
-		rule = (void *)g;
+		rule = ERR_CAST(g);
 		up_write_ref_node(&ft->node);
 		return rule;
 	}