Claude review: serdev: add rust private data to serdev_device

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

Patch Review

**Summary:** Adds a `void *rust_private_data` field to `struct serdev_device` for use by the Rust abstraction.

```c
+ * @rust_private_data:	Private data for the rust abstraction. This should
+ *			not be used by the C drivers.
  */
 struct serdev_device {
 	...
+	void *rust_private_data;
 };
```

This is a C-side change touching a core kernel structure. The field is needed because the Rust abstraction stores a `PrivateData` (with a `Completion` for probe/receive synchronization) that must be accessible from the `receive_buf` callback before drvdata is fully set up. This is the same pattern used by other subsystems.

**Minor:** The doc comment says "rust" lowercase — kernel convention in documentation tends to capitalize "Rust" (e.g., `Documentation/rust/`). Consider `@rust_private_data: Private data for the Rust abstraction.`

This needs an Ack from the serdev maintainer (Rob Herring) since it modifies the core C struct.

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