greybus: hd: fix host-device-removal race

Make sure to tear down the svc and flush any on-going hotplug processing
before removing the remaining interfaces.

This fixes crashes due to host-device removal racing with svc
hotplug/unplug processing (e.g. at "UniPro restart").

Testing Done:
Verified that this fixes crashes reproducible on SDB when unloading the
host-device driver module while generating hotplug/unplug events.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>