Ardino LMIC asserts after join in US915

[terrillmoore]

See mcci-catena#44

TTN now sends MCMD_LADR_CHP_125OFF and then MCMD_LADR (selecting channels 0..15, enabling 8..15).

The problem is in processing MCMD_LADR_CHP_125OFF. Although we set the mask for channels 64..71 with:

LMIC.channelMap[64/16] = chmap & 0x00FF;

But we then immediately mis-set the number of enabled 500 kHz channels (we don't consider the mask), either clearing to 0 or setting to 8.

We also need to handle ChMaskCtl == 5, which is used for subband control.