lapdm: Do not return an error when enqueuing a frame

If tx_ph_data_enqueue() is called, frames will be written into a queue,
if there is a pending frame or if polling of TX frames is used. In
this case the return value must be 0.

Sending a RSL_MT_UNIT_DATA_REQ from upper layer causes a call to
tx_ph_data_enqueue(). The return code is returned to the sender. The
sender must not get an error returned, if the message is enqueued.

Change-Id: Iaeaf7c66cb3cf5cc81bc8e15d468e8e7704c1407
diff --git a/src/gsm/lapdm.c b/src/gsm/lapdm.c
index 8ca1342..e8d9e78 100644
--- a/src/gsm/lapdm.c
+++ b/src/gsm/lapdm.c
@@ -362,7 +362,7 @@
 		*msgb_push(msg, 1) = link_id;
 		*msgb_push(msg, 1) = chan_nr;
 		msgb_enqueue(&dl->dl.tx_queue, msg);
-		return -EBUSY;
+		return 0;
 	}
 
 	osmo_prim_init(&pp.oph, SAP_GSM_PH, PRIM_PH_DATA,