Shuffle around the RTS/CTS rate/duration logic.

Shuffle around the RTS/CTS rate/duration logic.

* Turn ath_tx_calc_ctsduration() into a function that
  returns the ctsduration, or -1 for HT rates;
* add a printf() to ath_tx_calc_ctsduration() which will be
  very loud if somehow that function is called with an MCS
  rate;
* Add ath_tx_get_rtscts_rate() which returns the RTS/CTS
  rate to use for the given data rate, incl. the short
  preamble flag;
* Only call ath_tx_calc_ctsduration() for non-11n chipsets;
  11n chipsets don't require the rtscts duration to be
  calculated.