ACPI: fix hotplug race

The hotplug notification handler and drivers' notification handler all
run in one workqueue.  Before hotplug removes an acpi device, the
device driver's notification handler is already be recorded to run just
after global notification handler.  After hotplug notification handler
runs, acpica will notice a NULL notification handler and crash.

So now we run run hotplug in another workqueue and wait
for all acpi notication handlers finish.
This was found in battery hotplug, but actually all
hotplug can be affected.

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>