ath9k: move ar9100 version checking macros into a more appropriate place

All other version checking macros are in a common location within the
reg.h file. The AR_SREV_9100_OR_LATER macro is wrong currently, but will
be fixed with the next patch.

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>