CC: gsm48_cc_tx_release() must not remove callref

If after release has been sent, the call control layer waits for:
- reception of release complete
- or timeout
- or release of transaction (due to radio link failure)
In this case, an MNCC_REL_CNF is sent to upper layer. The callref must
still exist, so the upper layer can handle this confirm.

openbsc/src/libmsc/gsm_04_08.c

@@ -2300,8 +2300,6 @@ static int gsm48_cc_tx_release(struct gsm_trans *trans, void *arg)
 
 	gh->msg_type = GSM48_MT_CC_RELEASE;
 
-	trans->callref = 0;
-	
 	gsm48_stop_cc_timer(trans);
 	gsm48_start_cc_timer(trans, 0x308, GSM48_T308);