release channel on ERROR INDICATION

This was proposed by Andreas Eversberg.  I made it conditional on the T200
timer expired cause, as I'm not sure if we really should give up that quickly
on other errors such as just simply receiving an unsolicited response.

openbsc/src/abis_rsl.c

@@ -1106,6 +1106,9 @@ static int rsl_rx_rll_err_ind(struct msgb *msg)
 
 	DEBUGPC(DRLL, "cause=0x%02x", rlm_cause[1]);
 		
+	if (rlm_cause[1] == RLL_CAUSE_T200_EXPIRED)
+		return rsl_chan_release(msg->lchan);
+
 	return 0;
 }