smpp: fix return cause

Return cause 38 when default SMPP route is unavailable. This
is better than cause 1.

Change-Id: If3241d50a78fa611981e55fef6ae4c72b0a2a167
diff --git a/src/libmsc/smpp_smsc.h b/src/libmsc/smpp_smsc.h
index 4bee59b..fded236 100644
--- a/src/libmsc/smpp_smsc.h
+++ b/src/libmsc/smpp_smsc.h
@@ -126,8 +126,7 @@
 void smpp_esme_get(struct osmo_esme *esme);
 void smpp_esme_put(struct osmo_esme *esme);
 
-struct osmo_esme *
-smpp_route(const struct smsc *smsc, const struct osmo_smpp_addr *dest);
+int smpp_route(const struct smsc *smsc, const struct osmo_smpp_addr *dest, struct osmo_esme **emse);
 
 struct osmo_smpp_acl *smpp_acl_alloc(struct smsc *smsc, const char *sys_id);
 struct osmo_smpp_acl *smpp_acl_by_system_id(struct smsc *smsc,