Fix if conditiong/wrong variable name used (#24044)

diff --git a/src/platform/OpenThread/GenericThreadStackManagerImpl_OpenThread.cpp b/src/platform/OpenThread/GenericThreadStackManagerImpl_OpenThread.cpp
index 6e59b2e..6489f96 100644
--- a/src/platform/OpenThread/GenericThreadStackManagerImpl_OpenThread.cpp
+++ b/src/platform/OpenThread/GenericThreadStackManagerImpl_OpenThread.cpp
@@ -1162,7 +1162,7 @@
                     averageRssi.SetNonNull(neighInfo.mAverageRssi);
                 }
 
-                if (neighInfo.mAverageRssi == OT_RADIO_RSSI_INVALID)
+                if (neighInfo.mLastRssi == OT_RADIO_RSSI_INVALID)
                 {
                     lastRssi.SetNull();
                 }