ehci: Schedule async-bh when IAAD bit gets set

After the "ehci: Print a warning when a queue unexpectedly contains packets
on cancel" commit. Under certain reproducable conditions I was getting the
following message: "EHCI: Warning queue not empty on queue reset".

After aprox. 8 hours of debugging I've finally found the cause. The Linux EHCI
driver has an IAAD watchdog, to work around certain EHCI hardware sometimes
not acknowledging the doorbell at all. This watchdog has a timeout of 10 ms,
which is less then the time between 2 runs through the async schedule when
async_stepdown is at its highest value.

Thus the watchdog can trigger, after which Linux clears the IAAD bit and
re-uses the QH. IOW we were not properly detecting the unlink of the qh, due
to us missing (ignoring for more then 10 ms) the IAAD command, which triggered
the warning.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
