Any ideas why this should "freeze" up - note that the blue led on my board flashes a couple times then stays on. The USB device does not appear in `/dev/.cu*`

```rust
    loop {
        led.set_high();
        delay.delay_ms(100_u16);

        led.set_low();
        delay.delay_ms(100_u16);

        if !usb_dev.poll(&mut [&mut serial]) {
            continue;
        }

        serial.write(b"Hello, human...!").unwrap();
        serial.flush().unwrap();
    }
```