[subscriber] Update the current_bts field on imsi detach and LOC accept

When sending LOCATION UPDATING REQUEST Accept or when getting
a IMSI Detach update the gsm_bts of the gsm_subscriber.
diff --git a/src/gsm_04_08.c b/src/gsm_04_08.c
index 4b2ecdc..a9183de 100644
--- a/src/gsm_04_08.c
+++ b/src/gsm_04_08.c
@@ -584,7 +584,7 @@
 		return 0;
 
 	db_subscriber_alloc_tmsi(subscr);
-	subscr_update(subscr, bts);
+	subscr_update(subscr, bts, GSM_SUBSCRIBER_UPDATE_ATTACHED);
 
 	tmsi = strtoul(subscr->tmsi, NULL, 10);
 
@@ -806,10 +806,12 @@
 		break;
 	}
 
-	if (subscr)
+	if (subscr) {
+		subscr_update(subscr, msg->trx->bts,
+				GSM_SUBSCRIBER_UPDATE_DETACHED);
 		DEBUGP(DMM, "Subscriber: %s\n",
 		       subscr->name ? subscr->name : subscr->imsi);
-	else
+	} else
 		DEBUGP(DMM, "Unknown Subscriber ?!?\n");
 
 	put_lchan(msg->lchan);