hcd-0305, periodic and pci fixup
  
      - removes the pci dependency you mentioned in the
        rh_string code (friendlier to non-PCI HCs)
      - makes code match doc (8859-1 chars, not just ascii)
      - adds sanity checking for the periodic transfer interval,
        and forces it to a power-of-two (code can leave HCDs)
      - facilitates better IRQ sharing