ath9k: add MSI support

On new Intel platforms like ApolloLake, legacy interrupt mechanism
(INTx) is not supported, so WLAN modules are not working because
interrupts are missing, therefore this patch is to add MSI support to
ath9k.  With module paremeter "use_msi=1", ath9k driver would try to
use MSI instead of INTx.

Signed-off-by: Russell Hu <rhu@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>