ACPI: Discard invalid elements in _PSS package

Make sure that the _PSS list is sorted in
descending order by typical power dissipation.

http://bugzilla.kernel.org/show_bug.cgi?id=7880

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