nfit: Fix the check for a successful NFIT merge

Missed previously due to a lack of test coverage on a platform that
provided an valid response to _FIT.

Signed-off-by: Linda Knippers <linda.knippers@hpe.com>
Acked-by: Vishal Verma <vishal.l.verma@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>