ath9k: fix build with CONFIG_ATH9K_STATION_STATISTICS=y

kbuild bot reported that commit 592fa228 ("ath9k: remove struct
ath_atx_ac") broke the build when CONFIG_ATH9K_STATION_STATISTICS is y.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Fixes: 592fa228 ("ath9k: remove struct ath_atx_ac")
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>