iperf3: Support runing test with bidirectional data

Change-Id: Ia6f12c65fd3ea26d412997741cb8685a9646af43
diff --git a/doc/examples/4g_srsLTE/suites/4g/iperf3_dl.py b/doc/examples/4g_srsLTE/suites/4g/iperf3_dl.py
index bcc569a..66d75bb 100755
--- a/doc/examples/4g_srsLTE/suites/4g/iperf3_dl.py
+++ b/doc/examples/4g_srsLTE/suites/4g/iperf3_dl.py
@@ -21,7 +21,7 @@
 ue.connect(enb)
 
 iperf3srv.start()
-proc = iperf3cli.prepare_test_proc(True, ue.netns())
+proc = iperf3cli.prepare_test_proc(iperf3cli.DIR_DL, ue.netns())
 
 print('waiting for UE to attach...')
 wait(ue.is_connected, None)