Fixes name of calibration method for clarity.
Fixes name of calibration method for clarity.

The previously named startRfCalibration method had nothing to do with
the radio part of the module, it only pertained to the RC oscillator
used for the Listen Modes. This rename reflects that and adds some
explanation in the header file.

iwanders/plainRFM69,iwanders/plainRFM69,iwanders/plainRFM69