Up on call released release the channel and take the next item

Once a call is released, release the gsm_lchan and move to the
next item in pending_stations or wait for more work.
diff --git a/src/gsm_04_08.c b/src/gsm_04_08.c
index 918116d..4bdc5f7 100644
--- a/src/gsm_04_08.c
+++ b/src/gsm_04_08.c
@@ -537,6 +537,7 @@
 	struct gsm48_hdr *gh = msgb_l3(msg);
 	u_int8_t msg_type = gh->msg_type & 0xbf;
 	struct gsm_call *call = &msg->lchan->call;
+	struct gsm_network *network = msg->lchan->ts->trx->bts->network;
 	int rc = 0;
 
 	switch (msg_type) {
@@ -548,6 +549,8 @@
 		/* Answer from MS to RELEASE */
 		DEBUGP(DCC, "RELEASE COMPLETE (state->NULL)\n");
 		call->state = GSM_CSTATE_NULL;
+		if (network->call_released)
+			(*network->call_released)(msg->lchan);
 		break;
 	case GSM48_MT_CC_ALERTING:
 		DEBUGP(DCC, "ALERTING\n");