ath10k: Check tx_stats before use it

tx_stats will be freed and set to NULL before debugfs_sta node is
removed in station disconnetion process. So if read the debugfs_sta
node there may be NULL pointer error. Add check for tx_stats before
use it to resove this issue.

Signed-off-by: Yingying Tang <yintang@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>