log: abis_rsl: don't log 'error' when there is no error

The message 'RF Channel Release due error 0' keeps catching my eye because
it says 'error' even though the error code is zero, i.e. no error.
This shall end now.

Change-Id: Ie0b9d62e8ce85a096c963931e0ae5527b8dc490a
diff --git a/openbsc/src/libbsc/abis_rsl.c b/openbsc/src/libbsc/abis_rsl.c
index 4c8448e..d3d9f9e 100644
--- a/openbsc/src/libbsc/abis_rsl.c
+++ b/openbsc/src/libbsc/abis_rsl.c
@@ -803,7 +803,11 @@
 	msg->lchan = lchan;
 	msg->dst = lchan->ts->trx->rsl_link;
 
-	DEBUGP(DRSL, "%s RF Channel Release CMD due error %d\n", gsm_lchan_name(lchan), error);
+	if (error)
+		DEBUGP(DRSL, "%s RF Channel Release due to error: %d\n",
+		       gsm_lchan_name(lchan), error);
+	else
+		DEBUGP(DRSL, "%s RF Channel Release\n", gsm_lchan_name(lchan));
 
 	if (error) {
 		/*