[SeaBIOS] USB next steps

[Kevin O'Connor]

I just finished committing the EHCI support, and I thought it would be
good to review what's remaining wrt USB.  These are the things I'll be
looking into as I get time.

* refactoring the hub code.  The hub and root hub code is nearly
  identical.

* better disk timing and error support.  I occasionally see USB disk
  boot failures that appear to be due to minor timing differences.
  SeaBIOS should be more resilient to these types of failures.  Also,
  SeaBIOS should be able to reset the drive and retry on a failure.

* keyboard LED support.  Pressing capslock/numlock/etc. should cause
  the corresponding led to turn on.

* disk lun support.  Some devices are multi-lun - in particular, one
  of my multi-card reader devices shows up this way.

* disk writing support.  It's probably not very useful, but it's also
  not hard to implement.

* usb mouse support.  Also not very useful, but also not hard to
  implement.

There is no OHCI support for bulk devices, and I don't plan to add it.
I don't see a way to cleanly support bulk transfers from 16bit mode
with OHCI.  Since most people will have EHCI, I don't think it's worth
adding support.  Also, for the same reason, I don't plan to add
keyboard led control for ohci either.

-Kevin