Fix tests on return value from acpi_evaluate_integer().  Based on a patch by
Roel Kluin <roel.kluin@gmail.com> and incorporating suggestions from Len
Brown <lenb@kernel.org>.
Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Jonathan Woithe <jwoithe@physics.adelaide.edu.au>
Signed-off-by: Len Brown <len.brown@intel.com>