This makes the EHCI driver stop trying to update a live QH ... it's
not like OHCI, that can't be done safely because of a hardware race.
The fix:

    - Unlinks the QH before updating it; only the tail can safely be
      updated "live", not the queue head.  The async schedule (all
      control/bulk QHs) and periodic schedule (interrupt QH) work a
      bit differently ... high bandwidth transfers will hiccup.

    - Moves "update QH" and "clear toggle" logic into one new
      qh_refresh() routine, used in several places.

The race shows readily enough under load with the right hardware.
The controller silicon might be relatively slow, or maybe it's the
bus that's slow/busy:

	Host			Controller
	---			----------
				reads two TD pointers
	update two TD pointers
	wmb()
	activate QH
				reads rest of QH

Net result is that the HC treated old TD pointers as valid, and things
started misbehaving.  Busy controllers would misbehave worse; some systems
wouldn't notice more than a slowdown, especially with light USB loads.

This affects behavior in two cases.  The uncommon one is when an endpoint
gets an error and halts.  The more common one happens when the controller
runs off the end of its queue and overlays an inactive "dummy" TD into
the QH ... something the spec says shouldn't happen, but which more
silicon seems to be doing.  (Presumably to reduce DMA chatter.)
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>