ath9k_hw: simplify revision checks for AR9280

Since AR9280 v1.0 was never sold (and the initvals removed), v1.0 specific
revision checks can be removed and the 'v2.0 or later' check can be
simplified to a check for AR9280 or later.

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