This patch is the result of that discussion a short while back
to fix the "hub driver polls too quickly at high speed" bug.

    - redefines "interval" of usb_fill_int_urb() to be what
      the endpoint descriptor returns, and transparently
      does the log-to-linear conversion if it's high speed

    - (most of the patch by volume!!) moves declarations
      forward so testing dev->speed there will compile.

No driver changes were needed -- most drivers pass the
endpoint bInterval value already, and those that don't are
not dealing with high speed device quirks.

p.s. Includes an unrelated one-liner:  deletes export of
    usb_inc_dev_use() so it now shows up in kernel doc.
    Minor oversight in an earlier patch.