Extend the ANI code to implement basic channel survey support.

Extend the ANI code to implement basic channel survey support.

* Always call ar5416GetListenTime()
* Modify ar5416GetListenTime() to:
  + don't update the ANI state if there isn't any ANI state;
  + don't update the channel survey state if there's no active
    channel - just to be paranoid
  + copy the channel survey results into the current sample slot
    based on the current channel; then increment the sample counter
    and sample history counter.
* Modify ar5416GetMIBCyclesPct() to simply return a HAL_SURVEY_SAMPLE,
  rather than a set of percentages.  The ANI code wasn't using the
  percentages anyway.

TODO:

* Create a new function which fetches the survey results periodically
* .. then modify the ANI code to use the pre-fetched values rather than
  fetching them again
* Roll the 11n ext busy function from ar5416_misc.c to update all the
  counters, then do the result calculation
* .. then, modify the MIB counter routine to correctly fetch a snapshot -
  freeze the counters, fetch the values, then reset the counters.