This patch lets more devices hook up to USB 2.0 hubs, stuff
like keyboards, mice, hubs that hasn't worked yet:

- schedules full/low speed interrupt transactions
- tracks CSPLIT bandwidth for full/low speed interrupt
  transactions
- moves some bus bandwidth calculation out of the EHCI code
- makes the bandwidth calculation primitive public, and
  adds kerneldoc for it

It still takes a scheduling shortcut, placing at most one
interrupt transaction in a frame (vs potentially over 100),
but it should do for now.