LMIC validDR() doesn't check out-of-range

[terrillmoore]

If you pass an invalid value to validDR() [off end of table], it returns garbage. Unfortunately, MAC commands use DRs and assume validDR() will do the right thing. And the compliance test for EU sends invalid DRs (section 5.4).

[terrillmoore]

The implementation is inherently somewhat fragile. But the solution is to make validDR() a region-sensitive API, so that it co-resides with the implementation of _DR2RPS_CRC, allowing it to check against the size.

[terrillmoore]

Fixing this requires moving some material from lorabase.h to an LMIC-specific file. Those items (some static inline functions) actually already were LMIC-specific, but didn't really depend on the new region scheme... apparently. They're all in-scope for clients after #include "lmic.h", so we can't just rip them out. Moving them to lmic_bandplan.h will make things work.