ath5k: Allow user/driver to set txpower

* Now that we have regulatory control enable the driver to set
 txpower on hw

 * Also use txpower table offset so that we can match
 power range set by user/driver with indices on power table.

 Tested 2 different cards (a CM9 and an RF5112-based ubnt) and got
 the same output using a remote machine to measure per-packet rssi
 (conected the cards using attenuators). I also switched between
 various tx power levels and i saw an equal power change on the remote
 machine (so txpower changes as expected) and verified that we have
 the same output on each rate.

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