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` with a doc comment:

```c
 * @rust_private_data:	Private data for the rust abstraction. This should
 *			not be used by the C drivers.
```

**Design concern:** This adds a void pointer to a C struct solely for Rust's benefit. It's needed because `drvdata` is already used for the driver's own data (T), and the Rust abstraction layer needs a separate location for its internal `PrivateData` (a `Completion` + error flag for probe/receive synchronization). There's no compile-time enforcement preventing C drivers from touching this field.

**Minor:** The field is uninitialized in C (no `= NULL` in allocators). The Rust side sets it during probe before it's read anywhere, so this is fine in practice, but it might be worth considering whether the serdev core allocator (`serdev_device_alloc`) should zero-initialize it for safety. Actually, `kzalloc` is typically used, so it should be zeroed.

No blocking issues, but this design choice merits subsystem maintainer buy-in (and the cover letter appropriately asks Rob Herring about this).

---

---
Generated by Claude Code Patch Reviewer