Don't try to initialize mouse or keyboard if there's no port

[YtvwlD]

I had hoped that #1082 would be my last PS/2-related PR, but alas, the hardware I'm working with behaves differently whether it's being cold booted or rebooted from Linux. :(

There seems to be a case where the controller is detected as single channel, but resetting the mouse still succeeds. Setting the sampling rate then fails.

I hope this change makes sense. Input still doesn't work on the Optiplex, but I'm running into less errors on boot.