Fix initial channel randomization for the DECI band
Fix initial channel randomization for the DECI band

It seems there was a typo in a constant, causing the CENTI band starting
channel to be randomized twice, while the initial channel for the DECI
band was left unset (so presumably 0).

This fixes the code to properly randomize the channel again.

mcci-catena/arduino-lmic,mcci-catena/arduino-lmic,mcci-catena/arduino-lmic