ath9k: Fix AIC compilation error

AIC needs to be registered only when BTCOEX is enabled.
This fixes the error reported by kbuild:

>> ERROR: "ar9003_hw_attach_aic_ops" [drivers/net/wireless/ath/ath9k/ath9k_hw.ko] undefined!

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>