xhcid: Sleep for 2ms on every poll loop

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.

xhcid/src/xhci/irq_reactor.rs

@@ -105,7 +105,7 @@ impl IrqReactor {
     }
     // TODO: Configure the amount of time wait when no more work can be done (for IRQ-less polling).
     fn pause(&self) {
-        std::thread::yield_now();
+        std::thread::sleep(std::time::Duration::from_millis(2));
     }
     fn run_polling(mut self) {
         debug!("Running IRQ reactor in polling mode.");