pcuif: Improve BTS-supported CS/MCS handling

Take into account the MCS values supported by the BTS. In osmo-bts,
in general all MCS are enabled if "mode egprs" is selected in BSC,
and none otherwise.

Change-Id: Ie8f0215ba17da1e545e98bec9325c02f1e8efaea
diff --git a/src/bts.cpp b/src/bts.cpp
index 6a7960c..359f2d5 100644
--- a/src/bts.cpp
+++ b/src/bts.cpp
@@ -210,7 +210,7 @@
 	bts->fc_interval = 1;
 	bts->initial_cs_dl = bts->initial_cs_ul = 1;
 	bts->initial_mcs_dl = bts->initial_mcs_ul = 1;
-	bts->cs1 = 1;
+	bts->cs_mask = 1 << 0;  /* CS-1 always enabled by default */
 	bts->n3101 = 10;
 	bts->n3103 = 4;
 	bts->n3105 = 8;