Sleep for 2ms on every poll loop in xhcid

This significantly reduces cpu usage. On my laptop before this change a single core would be fully loaded while running at 3-4GHz. With this change it would only be loaded for about 50% while running at 1-2GHz. This improves battery lifetime while also preventing the fan from spinning up to a distracting level.

This shouldn't noticably affect latency given that most keyboards and mice don't support polling at 500Hz anyway.