enb_srs: add extra head room for max rate with 6 PRB in MIMO mode

the overhead with 6 PRB and MIMO is a bit higher when compared
to other PRBs resulting in lower achievable throughput

Change-Id: I63888435553bba4f7be88cc745e24472921a7fb4

src/osmo_gsm_tester/obj/enb_srs.py

@@ -252,8 +252,13 @@ class srsENB(enb.eNodeB):
             max_rate = max_phy_rate_tm1_ul[self.num_prb()]
 
         # MIMO only supported for Downlink
-        if downlink and self._txmode > 2:
-            max_rate *= 2
+        if downlink:
+            if self._txmode > 2:
+                max_rate *= 2
+
+            # For 6 PRBs the max throughput is significantly lower
+            if self._txmode >= 2 and self.num_prb() == 6:
+                max_rate *= 0.85
 
         return max_rate