ath9k: move update tx power to common

move ath_update_txpow to common to remove code duplication
in both ath9k & ath9k_htc.

Signed-off-by: Rajkumar Manoharan <rmanoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
3 files changed