Fix typo in EthernetTetheringTestBase

Bug: 361219545
Test: atest EthernetTetheringTest#testTetherClatBpfOffloadUdp
Change-Id: Ib1e48799f37b19390d304822ec55d5ad3615eaa5
diff --git a/Tethering/tests/integration/base/android/net/EthernetTetheringTestBase.java b/Tethering/tests/integration/base/android/net/EthernetTetheringTestBase.java
index d5c6a8e..423b9b8 100644
--- a/Tethering/tests/integration/base/android/net/EthernetTetheringTestBase.java
+++ b/Tethering/tests/integration/base/android/net/EthernetTetheringTestBase.java
@@ -975,7 +975,7 @@
     }
 
     protected String getUpstreamInterfaceName() {
-        if (mUpstreamReader == null) return null;
+        if (mUpstreamTracker == null) return null;
         return mUpstreamTracker.getTestIface().getInterfaceName();
     }