ath9k: move ath9k_hw_fbin2freq function to eeprom.h

Both eeprom.c and ar9003_eeprom.c has an indentical
'ath9k_hw_fbin2freq' function. Move the function to
a common place and remove the duplicates.

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