Issue(#108): Correct ADR behaviour on US915. Add a function to validate the datarate against a given channel mask. Correct the minimum number of 125kHz channels from 6 to 2.
Issue(#108): Correct ADR behaviour on US915. Add a function to validate the datarate against a given channel mask. Correct the minimum number of 125kHz channels from 6 to 2.

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