thunderbolt: Keep the domain reference while processing hotplug

thunderbolt: Keep the domain reference while processing hotplug

We process hotplug events in a workqueue that may run after the domain
has been removed by tb_domain_remove(). For example if user unloads the
driver while at the same time plugging  a device router we may have
scheduled tb_handle_hotplug() to run. Avoid possible UAF in this case by
taking the domain reference before scheduling the hotplug handler in
tb_queue_hotplug().

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>