ath9k_hw: clean up AR9003 EEPROM code

- add an inline function for getting the correct modal EEPROM struct
- remove unnecessary indirection through ath9k_hw_ar9300_get_eeprom
  access the relevant fields directly

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>