LMIC can get stuck with OP_TXRXPENDING if FSK is allowed for region

[terrillmoore]

v3.0.99 compliance gets stuck during the EU RX Timing Setup Request (test 13.1).

Although it looked like a missing interrupt, a long and painful investigation revealed that the function LMICbandplan_isFSK() was returning TRUE based on LMIC.rxsyms rather than LMIC.dndr. Sometimes LMIC.rxsyms was accidentally the wrong value, and we took the FSK path. The FSK path has a bug that causes it not to schedule the RX properly -- but even if it were bug free, that's the wrong path.