ath9k: introduce ath9k_hw_get_scaled_power helper

The computation of the scaled power value in
various eeprom files uses identical code. Move
that code into a helper function and use that
instead of code duplication.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>