This is a minor update to the patch I sent out about a week ago.
The key change is to use the I/O watchdog while doing ISO streaming.
Bernd Porr reports that a VT8235 system needs that; it seems like
IDE activity can interfere with the delivery of USB IRQs.


EHCI periodic scheduling updates.

 - Initial version of full speed ISO transaction support.  This
   should handle OUT transactions, such as those for usb speakers.
   For now, it's controlled using an EXPERIMENTAL config option:

   * I've run into interesting differences in how different USB 2.0
     hub silicon (the transaction translators) handle some older
     audio devices.  Needs more investigation.

   * Interrupt transfer scheduling doesn't yet cope well with schedules
     where every slot already has activity.  For now, don't plug in
     devices like hubs, mice, or keyboards while EHCI is streaming.

 - Protect freelist for highspeed ITDs, using spinlock.  Could be
   an issue for some drivers.

 - Kick in the I/O watchdog timer (5 msec) for periodic transfers.
   In this case, IDE activity on a VT8235 lost the IRQs which should
   have kept the ISO stream active.  Queues shorter than 5 msec are
   not going to work on all USB hosts.

 - Simplified the ISO scheduler:  doesn't attempt to re-schedule
   after lossage, or to short-circuit scanning.  (Rescheduling will
   probably come back later ... for now, the "hard" error here is
   highlighting problems that need attention.)