data tree UPDATE parse unknown op nodes as opaque

Instead of being struct and retuning an error.
Refs cesnet/libnetconf2#318
diff --git a/src/tree_data.c b/src/tree_data.c
index a5af00e..ccce0f7 100644
--- a/src/tree_data.c
+++ b/src/tree_data.c
@@ -549,7 +549,7 @@
         LY_CHECK_ARG_RET(ctx, format == LYD_XML, parent, parent->schema->nodetype & (LYS_RPC | LYS_ACTION), tree, !op,
                 LY_EINVAL);
     }
-    parse_opts = LYD_PARSE_ONLY | LYD_PARSE_STRICT;
+    parse_opts = LYD_PARSE_ONLY | LYD_PARSE_OPAQ;
     val_opts = 0;
 
     /* parse the data */