net: mqtt: Fix possible socket leak with websocket transport

In case underlying TCP/TLS connection is already down, the
websocket_disconnect() call is expected to fail, as it involves
communication. Therefore, mqtt_client_websocket_disconnect() should not
quit early in such cases, as it could lead to an underlying socket leak.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>
(cherry picked from commit 4625fa7)

subsys/net/lib/mqtt/mqtt_transport_websocket.c

@@ -174,8 +174,7 @@ int mqtt_client_websocket_disconnect(struct mqtt_client *client)
 
 	ret = websocket_disconnect(client->transport.websocket.sock);
 	if (ret < 0) {
-		NET_ERR("Websocket disconnect failed (%d)", ret);
-		return ret;
+		NET_DBG("Websocket disconnect failed (%d)", ret);
 	}
 
 	if (client->transport.type == MQTT_TRANSPORT_NON_SECURE_WEBSOCKET) {