ath9k: Fix rd_ext EEPROM capability for AR9285

AR9285 chipsets have a different EEPROM layout,
handle this appropriately when populating the rd_ext
capability.

Signed-off-by: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>