ath9k: fix null pointer dereference in ani monitor code

In 'ath9k_ani_reset' the 'ahp->ah_curani' will be initialized only
if 'DO_ANI(ah)' true. In 'ath9k_hw_ani_monitor' we are using
'ahp->ah_curani' unconditionally, and it will cause a NULL pointer
dereference on AR9100.

Changes-licensed-under: ISC

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