[link-quality] last RSSI ignore invalid RSSI (#2240)

* Add in filtering for invalid 127 RSSI in the last rssi field

src/core/thread/link_quality.cpp

@@ -154,6 +154,8 @@ void LinkQualityInfo::AddRss(int8_t aNoiseFloor, int8_t aRss)
 {
     uint8_t oldLinkQuality = kNoLinkQuality;
 
+    VerifyOrExit(aRss != OT_RADIO_RSSI_INVALID);
+
     mLastRss = aRss;
 
     if (mRssAverager.HasAverage())