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
(cherry picked from commit b5255ff34d24a2c9b85967e2dc00f84cd72406bf)
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;
     }