Lowering error log for optional TLV type in LLDP packet

Processing of the LLDP packet can proceed with no issue without the TLV and null cases are handled in sequent methods.
This shoudl not be treated as an error warning.

Change-Id: I3c201ca1c9daaf030fed6dbb09a6608527df283d
diff --git a/utils/misc/src/main/java/org/onlab/packet/ONOSLLDP.java b/utils/misc/src/main/java/org/onlab/packet/ONOSLLDP.java
index 9b379f6..ded5a90 100644
--- a/utils/misc/src/main/java/org/onlab/packet/ONOSLLDP.java
+++ b/utils/misc/src/main/java/org/onlab/packet/ONOSLLDP.java
@@ -252,7 +252,7 @@
             }
         }
 
-        log.error("Cannot find the port description tlv type.");
+        log.debug("Cannot find the port description tlv type.");
         return null;
     }