ath9k: Fix bugs in handling TX power

* Get power table offset from the EEPROM instead of using
  a hardcoded value of -5 if the EEPROM rev is >= 21.
* Add support in the 4k eeprom code for tx power offset
  in case we have a 4k AR9280 implementation.
* Fix tx power accuracy at high powers.

Signed-off-by: Senthil Balasubramanian <senthilkumar@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>