ath9k_hw: always enable 5 GHz fast clock for AR9280

All devices support this, but some disable it using an EEPROM flag
for some reason. Improves 5 GHz performance on those devices.

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