ACPI: fixes a false alarm from lockdep

fixes a false alarm from lockdep, as acpi hotplug workqueue waits other
workqueues.
http://bugzilla.kernel.org/show_bug.cgi?id=14553
https://bugzilla.kernel.org/show_bug.cgi?id=15521



Original-patch-from: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>