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/tbf_ul.cpp b/src/tbf_ul.cpp
index 43cd4f6..e4e4f79 100644
--- a/src/tbf_ul.cpp
+++ b/src/tbf_ul.cpp
@@ -167,7 +167,7 @@
 	ul_tbf->trx = trx;
 	/* The only one TS is the common, control TS */
 	ul_tbf->first_ts = ts;
-	ul_tbf->first_common_ts = ts;
+	ms_set_first_common_ts(ms, ts);
 	tbf_assign_control_ts(ul_tbf);
 	ul_tbf->m_ctrs = rate_ctr_group_alloc(ul_tbf, &tbf_ctrg_desc, next_tbf_ctr_group_id++);
 	ul_tbf->m_ul_egprs_ctrs = rate_ctr_group_alloc(ul_tbf,