samples: net: secure_mqtt: error code to enum

The error code `0x80` is replaced with its
corresponding `mqtt_suback_return_code` enum
value. This will make the code more readable, as
users of this example do not have to look up, what
`0x80` encodes.

Signed-off-by: Pályi Lőrinc <[email protected]>

Author: Lorl0rd

samples/net/secure_mqtt_sensor_actuator/src/mqtt_client.c

@@ -214,7 +214,7 @@ static void mqtt_event_handler(struct mqtt_client *const client, const struct mq
 		break;
 
 	case MQTT_EVT_SUBACK:
-		if (evt->result == 0x80) {
+		if (evt->result == MQTT_SUBACK_FAILURE) {
 			LOG_ERR("MQTT SUBACK error [%d]", evt->result);
 			break;
 		}