net: lwm2m: Lower log level for duplicate response

In networks with high latencies (like NB-IoT), it's quite common to
recieve duplicated response. It's not an error condition, a correct way
to handle it is to simply ignore the duplicate. Lower the log level for
this event, not to disturb users.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>

subsys/net/lib/lwm2m/lwm2m_engine.c

@@ -3711,7 +3711,7 @@ static void lwm2m_udp_receive(struct lwm2m_ctx *client_ctx,
 			lwm2m_reset_message(msg, true);
 		}
 	} else {
-		LOG_ERR("No handler for response");
+		LOG_DBG("No handler for response");
 	}
 }