ath9k: get EEPROM contents from platform data on AHB bus

On the AR913x SOCs we have to provide EEPROM contents via platform_data,
because accessing the flash via MMIO is not safe. Additionally different
boards may store the radio calibration data at different locations.

Changes-licensed-under: ISC

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Imre Kaloz <kaloz@openwrt.org>
Tested-by: Pavel Roskin <proski@gnu.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>