Don't use Crouch or Prone, rather use Go To Prone (I have it set as L Ctrl on my keypad). Tap it to crouch, hold it to go prone. It won't allow you to prone instantly but it's the quickest way to drop to the prone stance.
To paraphrase SixShooter, as you can't drop to prone from standing with a controller, you can't do so with a keyboard via XIM.