Cleaning up error handling for mss_freq L3

Change-Id: I799ed61062720822e8225efa8d0381a4c73a879c
Reviewed-on: http://ralgit01.raleigh.ibm.com/gerrit1/37283
Tested-by: PPE CI <ppe-ci+hostboot@us.ibm.com>
Reviewed-by: Thi N. Tran <thi@us.ibm.com>
Tested-by: Jenkins Server <pfd-jenkins+hostboot@us.ibm.com>
Reviewed-by: Jennifer A. Stofer <stofer@us.ibm.com>
Reviewed-on: http://ralgit01.raleigh.ibm.com/gerrit1/37550
Tested-by: FSP CI Jenkins <fsp-CI-jenkins+hostboot@us.ibm.com>
Reviewed-by: Sachin Gupta <sgupta2m@in.ibm.com>

src/import/chips/p9/procedures/xml/attribute_info/nest_attributes.xml

@@ -34,6 +34,13 @@
     Provided by the MRW.
   </description>
   <valueType>uint32</valueType>
+  <enum>
+    1600 = 1600,
+    1866 = 1866,
+    2000 = 2000,
+    2133 = 2133,
+    2400 = 2400
+  </enum>
   <writeable/>
   <platInit/>
 </attribute>