lorawan: add channels mask configuration.

Add channels mask configuration for LoRaWAN, creating a request for
 LoRaMAC. This change is needed because using LoRaWAN with ABP
activation without configuring channels mask will provide no acceptable
 channels on data transmit, causing data loss.

Signed-off-by: Jeferson Fernando <jeferson.santos@edge.ufal.br>