sanity checking and cleanup on device disconnect.
  
    - Splits apart usb_dec_dev_use(), for driver use, and
      usb_free_dev(), for hub/hcd use.  Both now have
      kerneldoc, and will BUG() if the refcount and the
      device tree get out of sync.  (Except for cleanup of
      root hub init errors, refcount must go to zero only
      at the instant disconnect processing completes.)
  
    - More usbcore-internal function declarations are
      now moved out of <linux/usb.h> into hcd.h
  
    - Driver-accessible refcounting is now inlined; minor
      code shrinkage, it's using atomic inc/dec instructions
      not function calls.

<note from greg k-h, there is still some work to be done with USB device
 reference counting, but this patch is a step in the right direction.>