Fix to MQTT client example: correct if() check. (#1115)

Another schoolboy error, this time in the MQTT client example: the if() check around uMqttClientSubscribe() was wrong; it just happened to work sometimes because the return value (the QOS of the subscription) can be 0, but it can also be 1 or 2. This is now fixed.

example/mqtt_client/mqtt_main.c

@@ -323,8 +323,11 @@ U_PORT_TEST_FUNCTION("[example]", "exampleMqttClient")
                     // uMqttClientSnSubscribeNormalTopic() instead and
                     // capture the returned MQTT-SN topic name for use with
                     // uMqttClientSnPublish() a few lines below
+                    // Note: >= in this case since the function
+                    // returns the QOS of the subscription, which
+                    // can be 0, 1 or 2
                     if (uMqttClientSubscribe(pContext, topic,
-                                             U_MQTT_QOS_EXACTLY_ONCE)) {
+                                             U_MQTT_QOS_EXACTLY_ONCE) >= 0) {
 
                         // Publish our message to our topic on the
                         // MQTT broker