> } else if (!data->has_subu) {
>         return dev_err_probe(dev, -EOPNOTSUPP,
>                              "No valid XTAL provided and SUBU mode not

Yeah, looks good to me. It is maybe a bit hidden that xtal == NULL in
the above case. I wonder if adding a comment above the return command

	/* xtal is NULL here */

will help understanding? But I'll leave it to you.