fix the logic for dropping logs

Change-Id: If3a1d5e39ae0305dbb6e286ab5049b921ffc3d35
diff --git a/aos/linux_code/logging/linux_logging.cc b/aos/linux_code/logging/linux_logging.cc
index c27e94b..b13a740 100644
--- a/aos/linux_code/logging/linux_logging.cc
+++ b/aos/linux_code/logging/linux_logging.cc
@@ -108,7 +108,7 @@
   if (__builtin_expect(dropped_messages > 0, false)) {
     ::aos::time::Time message_time =
         ::aos::time::Time(msg->seconds, msg->nseconds);
-    if (backoff_start - message_time < kDropBackoff) {
+    if (message_time - backoff_start < kDropBackoff) {
       ++dropped_messages;
       queue->FreeMessage(msg);
       return;