ACPI ac: update AC status upon sysfs query

http://marc.info/?l=linux-acpi&m=128855015826728&w=2
https://bugzilla.kernel.org/show_bug.cgi?id=21722



Tested_by: Seblu <seblu@seblu.net>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>