- Nitpickey bugfix to root hub config descriptors ... can't use
    the same one for high and full speed, since the encoding
    is different (255 ms FS == 0xff, 256 ms HS == 0x12).
  - Related, force period to 1/4 second rather than doing
    any sanity checking for the roothub timer (from Georg)
  - Don't "giveback" urbs on submit path errors (from Georg)
    ... means they don't get completion callbacks
  - Additional error checks on URB data (from Georg)
  - Uses <linux/completion.h> for unlink synchronization
  - The "already unlinking" error case is reported like other
    unlinking errors (not as success)
  - Ripped out urb->next handling ... it wasn't compatible
    with the ISO loop model, and at this point I believe it
    should be completely replaced with queuing urbs inside
    of the HCDs.  (Every HCD handles it for ISO, UHCI needs
    a magic flag to enable it for bulk ...)