ath9k_hw: remove dead code in the eeprom ops

The eeprom .set_addac function is only necessary for AR9160, remove it
from eeprom_4k.c and remove the dummy function from eeprom_9287.c

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