uhci: Don't crash on device disconnect

My recent uhci cleanup series has introduced a regression, where
qemu sometimes crashes on a device disconnect. The problem is that
the uhci code never checked for a device not / no longer existing, instead
it was relying on usb_handle_packet accepting a NULL device.

But since we now pass usb_handle_packet q->ep->dev, rather then just
a local dev variable, we crash as q->ep == NULL due to the device no longer
existing.

This patch fixes this. Note that this patch also improves over
the old behavior were we would:
1) create a queue for the device
2) create an async for the packet
3) have usb_handle_packet fail
4) destroy the async
5) wait for the queue to be idle for 32 frames
6) destroy the queue

Which was rather sub-optimal.

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