ath9k: add TPC to TX path for AR9002 based chips

Add TPC capability to TX descriptor path for AR9002 based chips. Scale
per-packet TX power according to eeprom power bias, power adjustments for
HT40 mode and open loop CCK rates. Cap per-packet TX power according to
TX power per-rate tables

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi83@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>