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/pcu_l1_if.cpp b/src/pcu_l1_if.cpp
index 577ba57..7068083 100644
--- a/src/pcu_l1_if.cpp
+++ b/src/pcu_l1_if.cpp
@@ -1071,7 +1071,7 @@
 	case GSM_MI_TYPE_IMSI:
 		req.mi_imsi = mi;
 		req.mi_imsi_present = true;
-		ms = bts_ms_by_imsi(bts, req.mi_imsi.imsi);
+		ms = bts_get_ms_by_imsi(bts, req.mi_imsi.imsi);
 		break;
 	default:
 		LOGP(DL1IF, LOGL_ERROR, "Unexpected MI type %u\n", mi.type);