mgcp: Return 0 in case transcoding is disabled.

In case transcoding is disabled no error has happened, return
a zero. This is fixing starting the nat without transcoding.
diff --git a/openbsc/src/mgcp/mgcp_protocol.c b/openbsc/src/mgcp/mgcp_protocol.c
index d1699e1..840bc14 100644
--- a/openbsc/src/mgcp/mgcp_protocol.c
+++ b/openbsc/src/mgcp/mgcp_protocol.c
@@ -955,7 +955,7 @@
 int mgcp_reset_transcoder(struct mgcp_config *cfg)
 {
 	if (!cfg->transcoder_ip)
-		return -1;
+		return 0;
 
 	static const char mgcp_reset[] = {
 	    "RSIP 1 13@mgw MGCP 1.0\r\n"