Claude review: rust: devres: return reference in `devres::register`

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

Patch Review

This changes `devres::register` from returning `Result` (unit) to `Result<&'a T>`, allowing callers to obtain a reference to the devres-managed data. The lifetime `'a` is tied to the `&'a Device<Bound>` borrow, which ensures the reference can't outlive the bound state (during which devres won't fire).

The safety argument is sound: `data_ptr` is obtained from the `Pin<KBox<T>>` heap allocation before the `KBox` is moved into `register_foreign`. The move only transfers ownership of the heap pointer — the allocation itself doesn't move, so `data_ptr` remains valid. After registration, devres owns the allocation and keeps it alive until unbind.

```rust
+    let data_ptr = &raw const *data;
+
     register_foreign(dev, data)
+        // SAFETY: `dev` is valid for the lifetime of 'a. As long as there is a reference to
+        // `Device<Bound>`, it is guaranteed that the device is not unbound and data has not been
+        // dropped. Thus `data_ptr` is also valid for the lifetime of 'a.
+        .map(|()| unsafe { &*data_ptr })
```

The existing callers (`cpufreq.rs`, `drm/driver.rs`) are updated to discard the new return value with `?; Ok(())`, preserving their existing behavior. Clean mechanical adaptation.

Reviewed-by quality: **Good.** Already has Viresh Kumar's Ack.

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