Certification now requires that we reject all attempts to change default channels

[terrillmoore]

The spec says this: LoRaWAN 1.0.3 section 5.6 (for NewChannelReq):

The acceptable range for ChIndex is N to 16.

In this case, N is our NUM_DEFAULT_CHANNELS. LMIC_setupChannel() is used for this, but it was less restrictive.

This is now tested in the EU Certification Test, section 5.2, and we fail because LMIC_setupChannel() is forgiving for changing the DRs of default channels.

Given our emphasis on compliance, rather than adding special code to handle MAC commands differently than the APIs, we'll simply tighten up the APIs.