Correct hsa_agent_iterate_isas return code for CPUs.

When no isa's are available no callbacks should be invoked.  This
is not an error and should return success.

Change-Id: Ie4048aa8cbe5c3fdf5431f6a865021549ecf8a13

runtime/hsa-runtime/core/runtime/hsa.cpp

@@ -1547,7 +1547,7 @@ hsa_status_t hsa_agent_iterate_isas(
 
   const Isa *isa_object = agent_object->isa();
   if (!isa_object) {
-    return HSA_STATUS_ERROR_INVALID_AGENT;
+    return HSA_STATUS_SUCCESS;
   }
 
   return callback(Isa::Handle(isa_object), data);