ath5k: Add tx power calibration support

* Add tx power calibration support
* Add a few tx power limits
* Hardcode default power to 12.5dB
* Disable TPC for now

v2: Address Jiri's comments

Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>