[nat] Make create_sccp_src_ref return the SCCP Connection.

Right now it was not possible to just find a connection, by returning
the connection that is created we will have direct access to it. It
will be used by the local connection handling.
diff --git a/openbsc/src/nat/bsc_nat.c b/openbsc/src/nat/bsc_nat.c
index 0fa5713..3beefc5 100644
--- a/openbsc/src/nat/bsc_nat.c
+++ b/openbsc/src/nat/bsc_nat.c
@@ -644,7 +644,7 @@
 		case SCCP_MSG_TYPE_CR:
 			if (bsc_nat_filter_sccp_cr(bsc, msg, parsed, &con_type) != 0)
 				goto exit3;
-			if (create_sccp_src_ref(bsc, parsed) != 0)
+			if (!create_sccp_src_ref(bsc, parsed))
 				goto exit2;
 			con = patch_sccp_src_ref_to_msc(msg, parsed, bsc);
 			con->con_type = con_type;