ath9k: cleanup AR5416 version checking macros

Currently we have two different versions of this macros. Because they
would have to do the same thing, we should simplify and merge them.

Changes-licensed-under: ISC

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Imre Kaloz <kaloz@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>