Add missing comment to vairable LastTxChannel
Add missing comment to vairable LastTxChannel

reixd/LoRaMac-node,reixd/LoRaMac-node,abouillot/LoRaMac-node,abouillot/LoRaMac-node,imec-idlab/LoRaMac-node,imec-idlab/LoRaMac-node