ACPICA: Fix to make _SST method optional

Fixes a problem introduced in 20080514 where the status of
execution of _SST is incorrectly returned to the caller. _SST
is optional, and if it is AE_NOT_FOUND, the exception should be
ignored.

http://www.acpica.org/bugzilla/show_bug.cgi?id=716

Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Lin Ming <ming.m.lin@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>