Claude review: gpu: nova-core: split BAR acquisition in unbind()

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

Patch Review

Changes `unbind()` from using `warn_on!` + `inspect` to an early-return pattern with `dev_err!`:
```rust
+        let Ok(bar) = self.bar.access(dev) else {
+            dev_err!(dev, "failed to acquire bar for driver unbinding\n");
+            return;
+        };
```

This is a behavior change: previously a BAR acquisition failure would trigger a `WARN_ON` (which is a kernel stacktrace + taint), now it logs an error and silently returns. For an unbind path this seems reasonable — a WARN_ON during unbind is unhelpful — but **the switch from WARN_ON to dev_err silently downgrades the severity.** This is probably intentional given subsequent patches will add more logic after the BAR acquisition, but worth noting.

No blocking issues.

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