ath9k: fix tx99 potential info leak

When the user sets count to zero the string buffer would remain
completely uninitialized which causes the kernel to parse its
own stack data, potentially leading to an info leak. In addition
to that, the string might be not terminated properly when the
user data does not contain a 0-terminator.

Signed-off-by: Miaoqing Pan <miaoqing@codeaurora.org>
Reviewed-by: Christoph Böhmwalder <christoph@boehmwalder.at>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>