New periodic calibration scheme needed for 11n parts that have
multiple algorithms and potentially collect multiple samples.
Instead of a single calibration interval we now have short and long
intervals; the long interval roughly corresponds to the previous
single interval. The short interval is used to speedup collection
of samples and happens much quicker. We make calls using the short
interval until we're told the calibration work is complete at which
point we fallback to the long interval. In addition there is a
much longer reset interval used to flush all calibration state and
cause everthing to start anew.

With these changes you can also disable calibration entirely by
setting the long interval to zero.