net: mqtt_sn: Verify result of transport initialization

The result of the transport init() function should be propagated to the
application, otherwise the initialization could fail silently.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>

Author: rlubos

subsys/net/lib/mqtt_sn/mqtt_sn.c

@@ -1000,6 +1000,8 @@ int mqtt_sn_client_init(struct mqtt_sn_client *client, const struct mqtt_sn_data
 			struct mqtt_sn_transport *transport, mqtt_sn_evt_cb_t evt_cb, void *tx,
 			size_t txsz, void *rx, size_t rxsz)
 {
+	int ret = 0;
+
 	if (!client || !client_id || !transport || !evt_cb || !tx || !rx) {
 		return -EINVAL;
 	}
@@ -1020,10 +1022,10 @@ int mqtt_sn_client_init(struct mqtt_sn_client *client, const struct mqtt_sn_data
 	k_work_init_delayable(&client->process_work, process_work);
 
 	if (transport->init) {
-		transport->init(transport);
+		ret = transport->init(transport);
 	}
 
-	return 0;
+	return ret;
 }
 
 void mqtt_sn_client_deinit(struct mqtt_sn_client *client)