Add a function to verify the RX frequency of window 2 for US band.
Add a function to verify the RX frequency of window 2 for US band.

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