USB: EHCI: unlink one async QH at a time

This patch (as1648) fixes a regression affecting nVidia EHCI
controllers.  Evidently they don't like to have more than one async QH
unlinked at a time.  I can't imagine how they manage to mess it up,
but at least one of them does.

The patch changes the async unlink logic in two ways:

	Each time an IAA cycle is started, only the first QH on the
	async unlink list is handled (rather than all of them).

	Async QHs do not all get unlinked as soon as they have been
	empty for long enough.  Instead, only the last one (i.e., the
	one that has been on the schedule the longest) is unlinked,
	and then only if no other unlinks are in progress at the time.

This means that when multiple QHs are empty, they won't be unlinked as
quickly as before.  That's okay; it won't affect correct operation of
the driver or add an excessive load.  Multiple unlinks tend to be
relatively rare in any case.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Reported-and-tested-by: Piergiorgio Sartor <piergiorgio.sartor@nexgo.de>
Cc: stable <stable@vger.kernel.org> # 3.6
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>