Claude review: samples: rust: add Rust serial device bus sample device driver

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

Patch Review

A clean sample driver demonstrating OF and ACPI device table matching, baudrate configuration from firmware properties, parity/flow control setup, and an echo-back receive callback.

**Probe error handling for `set_baudrate`:**
```rust
if sdev
    .set_baudrate(
        dev.fwnode()
            .and_then(|fwnode| fwnode.property_read(c"baudrate").optional())
            .unwrap_or(115200),
    )
    .is_err()
{
    return Err(EINVAL);
}
```
`set_baudrate` returns `Err(actual_baudrate)` when the hardware can't match the requested rate. Treating any mismatch as a hard error is fine for a sample, though real drivers may want to accept similar rates. The `if .is_err() { return Err(EINVAL) }` pattern could be simplified, but again, fine for illustration purposes.

**Receive callback silently discards write errors:**
```rust
fn receive<'bound>(
    sdev: &'bound serdev::Device<Bound>,
    _this: Pin<&Self>,
    data: &[u8],
) -> usize {
    let _ = sdev.write_all(data, serdev::Timeout::Max);
    data.len()
}
```
The `let _ =` discarding the `Result` is acceptable for a sample echo driver but would be a concern in production code. Worth a brief inline comment to note the intentional discard, since `let _ =` on a `Result` sometimes triggers lints.

**Kconfig entry:** Clean, correctly uses `select RUST_SERIAL_DEV_BUS_ABSTRACTIONS` to pull in the dependency.

---
Generated by Claude Code Patch Reviewer