ath9k: configure TPC through mac80211/cfg80211 API

Enable/disable per packet Transmit Power Control (TPC) using mac80211/cfg80211
API. In particular TPC is enabled if value passed in nl80211_tx_power_setting
is set to NL80211_TX_POWER_AUTOMATIC or NL80211_TX_POWER_LIMITED whereas
it is disabled if nl80211_tx_power_setting is NL80211_TX_POWER_FIXED.
Currently TPC is supported just by AR9003 based chips