fixup for Osmocom_CTRL_Functions: rate counters in bulk

At the time of writing Ief0d9b096feeee7d37b5f2429dd3e80de0161806 I wasn't aware
of the 'inout' keyword, which allows to pass the counter list by reference.

Rather modify the counter lists in-place. Instead of requiring

  list := f_counter_name_vals_add(list, ...)

rather implement by directly modifying list:

  f_counter_name_vals_add(list, ...)

Change-Id: I85ac56b042fe4bb1db392c1f451c8e900582cc2a
diff --git a/bsc/BSC_Tests.ttcn b/bsc/BSC_Tests.ttcn
index facb001..f25c75d 100644
--- a/bsc/BSC_Tests.ttcn
+++ b/bsc/BSC_Tests.ttcn
@@ -197,7 +197,7 @@
 }
 
 private function f_ctrs_msc_add(integer msc_nr, charstring countername, integer val := 1) runs on test_CT {
-	g_ctr_msc := f_counter_name_vals_list_add(g_ctr_msc, msc_nr, countername, val);
+	f_counter_name_vals_list_add(g_ctr_msc, msc_nr, countername, val);
 }
 
 /*  f_ctrs_msc_init();