ath9k_hw: clean up hardware revision checks

- AR_SREV_5416_20_OR_LATER is always true, remove it
- AR_SREV_9280_20_OR_LATER is always true within eeprom_4k.c and eeprom_9287.c
- (AR_SREV_9271 || AR_SREV_9285) is always true in eeprom_4k.c

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