Claude review: serdev: add rust private data to serdev_device

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Patch Review

This patch adds a `void *rust_private_data` field to `struct serdev_device` for use by the Rust abstraction's probe synchronization mechanism.

**Design concern**: Adding a Rust-specific field to a core C struct is an intrusive approach. Every `struct serdev_device` instance (including those used purely by C drivers) will carry this extra pointer. While the memory cost is negligible (8 bytes on 64-bit), this sets a precedent. An alternative would be to use a side table (e.g., an IDR or XArray keyed by device pointer), but that adds complexity. This design choice should be explicitly blessed by the serdev maintainer (Rob Herring).

**Documentation fix**: The patch also fixes the existing kerneldoc by adding missing colons to `@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
```

This kerneldoc fix should ideally be a separate patch to keep concerns separated, but it's minor.

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