Another cleanup patch:

   - removes more usbcore-internal symbols from sight of device drivers
       * some are only for the uhci driver
       * most are shared just a bit more widely
   - DEVNUM_ROUND_ROBIN is no longer an internal option
   - usb_root_hub_string() gone, per the FIXME
   - various kerneldoc fixes and updates
   - uses legal value (en-us) for root hub strings
   - hub driver only shows port fixed/removable for compound devices