fix: use MQTT_HELPER_SECONDARY_SEC_TAG

Author: coderbyheart

asset-tracker-cloud-firmware-azure.conf

@@ -15,7 +15,7 @@ CONFIG_EXTERNAL_SENSORS_IMPACT_DETECTION=y
 
 # Enable the secondary sec tag which should contain the Digicert root certificate
 # See https://techcommunity.microsoft.com/t5/internet-of-things-blog/azure-iot-tls-critical-changes-are-almost-here-and-why-you/ba-p/2393169
-CONFIG_AZURE_IOT_HUB_SECONDARY_SEC_TAG=12
+CONFIG_MQTT_HELPER_SECONDARY_SEC_TAG=12
 
 ## Disable FOTA TLS
 ## Download via HTTP is safe enough since firmware images are signed