Claude review: serdev: add rust private data to serdev_device

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

Patch Review

Adds `void *rust_private_data` to `struct serdev_device`:

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

This is the most debatable design choice in the series. It adds a pointer to every `serdev_device` instance, even when no Rust driver is present. The alternative would be using a side data structure (e.g., an IDR/XArray lookup), but that would add complexity and overhead in the receive path.

The doc update fixes the missing colons on `@write_comp` and `@write_lock`:
```c
- * @write_comp	Completion used by serdev_device_write() internally
- * @write_lock	Lock to serialize access when writing data
+ * @write_comp:	       Completion used by serdev_device_write() internally
+ * @write_lock:	       Lock to serialize access when writing data
```
That's a nice cleanup, but the re-alignment to very wide column positions (tabs to column ~24) changes the style relative to the other fields. Might attract style comments from the serdev maintainer.

The field is not initialized anywhere, but `serdev_device_alloc` uses `kzalloc` for the controller, and the device is allocated via `device_register`, so the field should be zero-initialized. Still, it would be more defensive to document or assert that assumption.

This patch is missing a `Signed-off-by` or `Acked-by` from Rob Herring (the serdev maintainer), which is important since it modifies a core C header he maintains.

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