wifi: Fix rate used to send BlockAckReq frames

Fixes a regression introduced with 1a8b3718

examples/wireless/wifi-simple-ht-hidden-stations.cc

@@ -25,6 +25,7 @@
 #include "ns3/ipv4-address-helper.h"
 #include "ns3/log.h"
 #include "ns3/mobility-helper.h"
+#include "ns3/rng-seed-manager.h"
 #include "ns3/ssid.h"
 #include "ns3/string.h"
 #include "ns3/udp-client-server-helper.h"
@@ -64,6 +65,9 @@ main(int argc, char* argv[])
     double minExpectedThroughput = 0;
     double maxExpectedThroughput = 0;
 
+    RngSeedManager::SetSeed(1);
+    RngSeedManager::SetRun(5);
+
     CommandLine cmd(__FILE__);
     cmd.AddValue("nMpdus", "Number of aggregated MPDUs", nMpdus);
     cmd.AddValue("payloadSize", "Payload size in bytes", payloadSize);