ath5k: Extend rate_duration

 * Extend ieee80211_generic_frame_duration to support the various
 bwmodes.

 * Better document what's going on with ack bitrates and update
 write_rate_duration to support the standard ack bitrates (when
 we don't set the high bit).

 * Get rid of set_ack_bitrate_high and introduce a flag on ath5k_hw
 for this (we only called the function on init anyway so there is no
 difference).

Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>

Signed-off-by: John W. Linville <linville@tuxdriver.com>