bts: Rename bts_ms_by_{tlli,imsi} -> bts_get_ms_by_{tlli,imsi}

While at it, put them together and mark bts param as const.
This is a preparation for next patch.

Change-Id: Iad8aec4424f1f23cd4d02a14c4f9ec1b9fdb1f75
diff --git a/src/gprs_bssgp_pcu.c b/src/gprs_bssgp_pcu.c
index 656342e..2c5a97a 100644
--- a/src/gprs_bssgp_pcu.c
+++ b/src/gprs_bssgp_pcu.c
@@ -244,9 +244,9 @@
 	 * target MS is using. */
 	llist_for_each_entry(bts, &the_pcu->bts_list, list) {
 		/* TODO: Match by TMSI before IMSI if present?! */
-		ms = bts_ms_by_tlli(bts, req.tlli, req.tlli);
+		ms = bts_get_ms_by_tlli(bts, req.tlli, req.tlli);
 		if (!ms && req.mi_imsi_present)
-			ms = bts_ms_by_imsi(bts, req.mi_imsi.imsi);
+			ms = bts_get_ms_by_imsi(bts, req.mi_imsi.imsi);
 		bts_add_paging(bts, &req, ms);
 	}