enb,ms: refactor base_rate selection for ZMQ

Use tailored sample rate only use for 6, 50 and 75 PRB. For all other bandwidths,
we use the full LTE rate and downsample.
With 5.76e6 as base sample rate, for example, we had some issue during the tests like:

[zmq] Error: tx time is 0.178 ms in the past (7833600 < 7835648)
[zmq] Error: tx time is 0.011 ms in the past (7835520 < 7835648)

Using a higher sample rate seems to work better.

Change-Id: I53678587b2c5e0e5dccb1b55a328ad2f1a97fc4c

src/osmo_gsm_tester/obj/enb.py

@@ -142,18 +142,14 @@ class eNodeB(log.Origin, metaclass=ABCMeta):
 
     #reference: srsLTE.git srslte_symbol_sz()
     def num_prb2symbol_sz(self, num_prb):
-        if num_prb <= 6:
+        if num_prb == 6:
             return 128
-        if num_prb <= 15:
-            return 256
-        if num_prb <= 25:
-            return 384
-        if num_prb <= 50:
+        if num_prb == 50:
             return 768
-        if num_prb <= 75:
+        if num_prb == 75:
             return 1024
-        if num_prb <= 110:
-            return 1536
+        return 1536
+
         raise log.Error('invalid num_prb %r', num_prb)
 
     def num_prb2base_srate(self, num_prb):

src/osmo_gsm_tester/obj/ms_amarisoft.py

@@ -45,8 +45,6 @@ def num_prb2symbol_sz(num_prb):
         return 128
     if num_prb <= 15:
         return 256
-    if num_prb <= 25:
-        return 384
     if num_prb <= 50:
         return 768
     if num_prb <= 75:

src/osmo_gsm_tester/obj/ms_srs.py

@@ -52,19 +52,13 @@ def on_register_schemas():
 
 #reference: srsLTE.git srslte_symbol_sz()
 def num_prb2symbol_sz(num_prb):
-    if num_prb <= 6:
+    if num_prb == 6:
         return 128
-    if num_prb <= 15:
-        return 256
-    if num_prb <= 25:
-        return 384
-    if num_prb <= 50:
+    if num_prb == 50:
         return 768
-    if num_prb <= 75:
+    if num_prb == 75:
         return 1024
-    if num_prb <= 110:
-        return 1536
-    raise log.Error('invalid num_prb %r', num_prb)
+    return 1536
 
 def num_prb2base_srate(num_prb):
     return num_prb2symbol_sz(num_prb) * 15 * 1000