enb: incorporate number of carriers into ue_max_rate() API

this allows the enb to calculate the max rate in a single
spot rather than each test individually

Change-Id: Id225a986e0b271cf9c057c74c9aefd6c7f3d39d6
diff --git a/sysmocom/suites/4g/iperf3_dl.py b/sysmocom/suites/4g/iperf3_dl.py
index 0d8bfcd..6f79076 100755
--- a/sysmocom/suites/4g/iperf3_dl.py
+++ b/sysmocom/suites/4g/iperf3_dl.py
@@ -20,8 +20,7 @@
 
 ue.connect(enb)
 
-max_rate = enb.ue_max_rate(downlink=True)
-max_rate *= ue.num_carriers
+max_rate = enb.ue_max_rate(downlink=True, num_carriers=ue.num_carriers)
 
 iperf3srv.start()
 proc = iperf3cli.prepare_test_proc(iperf3cli.DIR_DL, ue.netns(), bitrate=max_rate)