ath9k: fix tx99 bus error

commit bde717ab upstream.

The hard coded register 0x9864 and 0x9924 are invalid
for ar9300 chips.

Signed-off-by: Miaoqing Pan <miaoqing@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>