Changes to enable ACKs in mixed RFM69 / RFM12B environment

These are the matching changes made while testing the RFM12B compatibility changes.  These do not seem to be compatibility related but I found them while testing the compatibility code.

See discussion at LowPowerLab forum: http://lowpowerlab.com/forum/index.php/topic,209.15.html

Thanks,
  - Chip