USB: EHCI: fix regression in QH unlinking

This patch (as1670) fixes a regression caused by commit
6402c796

 (USB: EHCI: work around
silicon bug in Intel's EHCI controllers).  The workaround goes through
two IAA cycles for each QH being unlinked.  During the first cycle,
the QH is not added to the async_iaa list (because it isn't fully gone
from the hardware yet), which means that list will be empty.

Unfortunately, I forgot to update the IAA watchdog timer routine.  It
thinks that an empty async_iaa list means the timer expiration was an
error, which isn't true any more.  This problem didn't show up during
initial testing because the controllers being tested all had working
IAA interrupts.  But not all controllers do, and when the watchdog
timer expires, the empty-list check prevents the second IAA cycle from
starting.  As a result, URB unlinks never complete.  The check needs
to be removed.

Among the symptoms of the regression are processes stuck in D wait
states and hangs during system shutdown.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Reported-and-tested-by: Stephen Warren <swarren@wwwdotorg.org>
Reported-and-tested-by: Sven Joachim <svenjoac@gmx.de>
Reported-by: Andreas Bombe <aeb@debian.org>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>