Clean up after RTT operations finished. (1/2)

Bug: 36185358

Test: Manual with RTT app.

Change-Id: Ic9c9d020acaf384664a9881ec2806f941b7a0c1c
diff --git a/wifi/1.0/default/wifi_legacy_hal.cpp b/wifi/1.0/default/wifi_legacy_hal.cpp
index ba57ba7..225dadd 100644
--- a/wifi/1.0/default/wifi_legacy_hal.cpp
+++ b/wifi/1.0/default/wifi_legacy_hal.cpp
@@ -164,6 +164,7 @@
   const auto lock = hidl_sync_util::acquireGlobalLock();
   if (on_rtt_results_internal_callback) {
     on_rtt_results_internal_callback(id, num_results, rtt_results);
+    on_rtt_results_internal_callback = nullptr;
   }
 }