tbf: Make tbf_ms() param const

Change-Id: I041c564b15d17d05ce97ea0085fcd9192a346578
diff --git a/src/tbf.cpp b/src/tbf.cpp
index d2d55f2..0fec476 100644
--- a/src/tbf.cpp
+++ b/src/tbf.cpp
@@ -1165,7 +1165,7 @@
 	return &tbf->m_bts_list.list;
 }
 
-struct GprsMs *tbf_ms(struct gprs_rlcmac_tbf *tbf)
+struct GprsMs *tbf_ms(const struct gprs_rlcmac_tbf *tbf)
 {
 	return tbf->ms();
 }
diff --git a/src/tbf.h b/src/tbf.h
index d616076..983d38c 100644
--- a/src/tbf.h
+++ b/src/tbf.h
@@ -196,7 +196,7 @@
 void tbf_set_ms(struct gprs_rlcmac_tbf *tbf, struct GprsMs *ms);
 struct llist_head *tbf_ms_list(struct gprs_rlcmac_tbf *tbf);
 struct llist_head *tbf_bts_list(struct gprs_rlcmac_tbf *tbf);
-struct GprsMs *tbf_ms(struct gprs_rlcmac_tbf *tbf);
+struct GprsMs *tbf_ms(const struct gprs_rlcmac_tbf *tbf);
 bool tbf_timers_pending(struct gprs_rlcmac_tbf *tbf, enum tbf_timers t);
 void tbf_free(struct gprs_rlcmac_tbf *tbf);
 struct gprs_llc *tbf_llc(struct gprs_rlcmac_tbf *tbf);