Claude review: gpu: nova-core: use warn_on_err macro

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

Patch Review

**Status: Looks good**

Clean refactor of the `unbind` method:

```rust
-        kernel::warn_on!(self
-            .bar
-            .access(dev)
-            .inspect(|bar| self.sysmem_flush.unregister(bar))
-            .is_err());
+        let Ok(bar) = kernel::warn_on_err!(self.bar.access(dev)) else {
+            return;
+        };
+
+        self.sysmem_flush.unregister(bar);
```

This is a meaningful improvement. The old code used `inspect` to perform a side-effect (unregister) inside a chain, which is harder to read and doesn't allow adding more steps between `bar.access()` and the end of the function. The new early-return pattern is idiomatic Rust and prepares for the additional unload logic added in patches 5 and 6.

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