Default channels and mask wrong

[emanuele-dedonatis]

https://github.com/HelTecAutomation/ESP32_LoRaWAN/blob/7254947a32ffafbe437db810cbdf9a5f81cac26b/src/ESP32_LoRaWAN.cpp#L434

This line adds NOT-DEFAULT channels provided by LoRaWAN specifications. It should not be called.

EU868, for example, has only 3 default channels (EU868_LC1, EU868_LC2, EU868_LC3). This function will add EU868_LC4, EU868_LC5, EU868_LC6, EU868_LC7, EU868_LC8 and it's wrong. Optional channels will be sent by the network server in the CFList of the join-accept message OR through MAC commands.