[rsl] Introduce a method to set the state of the lchan

Setting the state through a dedicated method allows us to
track the state transitions and check if they are done in
a proper way.
diff --git a/openbsc/src/handover_logic.c b/openbsc/src/handover_logic.c
index 7fb0b13..7352d64 100644
--- a/openbsc/src/handover_logic.c
+++ b/openbsc/src/handover_logic.c
@@ -229,7 +229,7 @@
 	/* update lchan pointer of transaction */
 	trans_lchan_change(&ho->old_lchan->conn, &new_lchan->conn);
 
-	ho->old_lchan->state = LCHAN_S_INACTIVE;
+	rsl_lchan_set_state(ho->old_lchan, LCHAN_S_INACTIVE);
 	lchan_auto_release(ho->old_lchan);
 
 	/* do something to re-route the actual speech frames ! */