Move first_common_ts from gprs_rlcmac_tbf to GprsMs

The field contains a common value between the 2 active TBFs of the MS,
so it makes no sense to have them duplicated on each TBF. It can be
sanely stored in the MS object.

Change-Id: I8df01a99ccbfaf7a442ade5000ee282bd638fbba
diff --git a/src/gprs_rlcmac_ts_alloc.cpp b/src/gprs_rlcmac_ts_alloc.cpp
index b5757a7..026ad6e 100644
--- a/src/gprs_rlcmac_ts_alloc.cpp
+++ b/src/gprs_rlcmac_ts_alloc.cpp
@@ -411,8 +411,9 @@
 
 	tbf->trx = trx;
 	/* the only one TS is the common TS */
-	tbf->first_ts = tbf->first_common_ts = ts;
+	tbf->first_ts = ts;
 	ms_set_reserved_slots(ms, trx, 1 << ts, 1 << ts);
+	ms_set_first_common_ts(ms, ts);
 
 	tbf->upgrade_to_multislot = false;
 	bts_do_rate_ctr_inc(bts, CTR_TBF_ALLOC_ALGO_A);
@@ -945,9 +946,8 @@
 	/* Step 4: Update MS and TBF and really allocate the resources */
 
 	update_ms_reserved_slots(trx, ms, reserved_ul_slots, reserved_dl_slots, ul_slots, dl_slots);
-
+	ms_set_first_common_ts(ms, first_common_ts);
 	tbf->trx = trx;
-	tbf->first_common_ts = first_common_ts;
 	tbf->first_ts = first_ts;
 
 	if (tbf->direction == GPRS_RLCMAC_DL_TBF)