[simulators.gateway] Fix error in Unmarshal function
[simulators.gateway] Fix error in Unmarshal function

LoRaWanSoFa/ttn,LoRaWanSoFa/ttn,TheThingsNetwork/ttn,mgranberry/ttn,jvanmalder/ttn,jvanmalder/ttn,mgranberry/ttn,TheThingsNetwork/ttn,mgranberry/ttn,jvanmalder/ttn,TheThingsNetwork/ttn