Don't return an error to caller if we are out of lchan on CHAN RQD

This is a normal situation under high load: The BTS is out of resources
and the BSC cannot allocate any dedicated channel.  We may send an IMM
ASS REJECT back to the MS.  All well within normal parameters, not an
error that needs to be reported back to the caller, who propagates it
all the way up to libosmo-abis.

openbsc/src/libbsc/abis_rsl.c

@@ -1448,7 +1448,7 @@ static int rsl_rx_chan_rqd(struct msgb *msg)
 		/* FIXME gather multiple CHAN RQD and reject up to 4 at the same time */
 		if (bts->network->T3122)
 			rsl_send_imm_ass_rej(bts, 1, rqd_ref, bts->network->T3122 & 0xff);
-		return -ENOMEM;
+		return 0;
 	}
 
 	if (lchan->state != LCHAN_S_NONE)