Change log level to prevent warning flood in port table polling

Change-Id: I7a5404f17b853adddbe8d9518e6777ad29942d83
diff --git a/drivers/optical/src/main/java/org/onosproject/driver/optical/power/OplinkPowerConfigUtil.java b/drivers/optical/src/main/java/org/onosproject/driver/optical/power/OplinkPowerConfigUtil.java
index 45e2c58..1b2a0d1 100644
--- a/drivers/optical/src/main/java/org/onosproject/driver/optical/power/OplinkPowerConfigUtil.java
+++ b/drivers/optical/src/main/java/org/onosproject/driver/optical/power/OplinkPowerConfigUtil.java
@@ -400,7 +400,8 @@
         }
         String power = port.annotations().value(annotation);
         if (power == null) {
-            log.warn("Cannot get {} from port {}.", annotation, portNum);
+            // Do not need warning here for port polling.
+            log.debug("Cannot get {} from port {}.", annotation, portNum);
             return null;
         }
         return Long.valueOf(power);