ath9k: Handle txpower changes even when TPC is disabled

When TPC is disabled IEEE80211_CONF_CHANGE_POWER event can be handled to
reconfigure HW's maximum txpower.

This fixes 0dBm txpower setting when user attaches to an interface for
the first time with the following scenario:

ieee80211_do_open()
    ath9k_add_interface()
        ath9k_set_txpower() /* Set TX power with not yet initialized
                               sc->hw->conf.power_level */

    ieee80211_hw_config() /* Iniatilize sc->hw->conf.power_level and
                             raise IEEE80211_CONF_CHANGE_POWER */

    ath9k_config() /* IEEE80211_CONF_CHANGE_POWER is ignored */

This issue can be reproduced with the following:

  $ modprobe -r ath9k
  $ modprobe ath9k
  $ wpa_supplicant -i wlan0 -c /tmp/wpa.conf &
  $ iw dev /* Here TX power is either 0 or 3 depending on RF chain */
  $ killall wpa_supplicant
  $ iw dev /* TX power goes back to calibrated value and subsequent
              calls will be fine */

Fixes: 283dd119 ("ath9k: add per-vif TX power capability")
Cc: stable@vger.kernel.org
Signed-off-by: Remi Pommarel <repk@triplefau.lt>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>