LMIC_clrTxData can clobber join

[terrillmoore]

Calling LMIC_clrTxData at the wrong point in a join cycle can clobber the join. Don't clear TXRXPEND unless it's due to the TXDATA bit being set. This causes #445, because the compliance sketch was lucky enough to call at the wrong time.

[terrillmoore]

With this fix, things seem to be stable again (after the timing changes).