Claude review: serdev: add rust private data to serdev_device

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

```c
+	void *rust_private_data;
```

This adds a `void *rust_private_data` field to `struct serdev_device` in the C header. The field is documented as "Private data for the rust abstraction. This should not be used by the C drivers."

**Question:** Adding a field to a C struct that is only used by Rust is somewhat unusual. Normally bus subsystems provide a mechanism like `dev_set_drvdata()` for private data. The commit message says this is "required by the rust abstraction" but it would benefit from a brief explanation of *why* the existing `drvdata` mechanism isn't sufficient (presumably because `drvdata` is used for `T::Data` and the abstraction needs a second private data slot for the `PrivateData` mutex). The serdev maintainer (Rob Herring, being replaced in patch 5) should weigh in.

**Minor:** The doc comment reformatting (re-aligning the `@` entries with tabs) is a cosmetic change bundled with the functional change. It's harmless but slightly noisy.

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