CHROMIUM: WAR: ath10k: Check tx_stats before use it

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

BUG=b:132147953
TEST=None

Change-Id: I463f6445ddab625ba2b39688ea1168bbe192d8a7
CRs-Fixed: 24418492
Signed-off-by: Yingyingtang <yintang@codeaurora.org>
Signed-off-by: Vamsi Singamsetty <vamssi@codeaurora.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/1597059


Reviewed-by: Zhihong Yu <zhihongyu@chromium.org>
Reviewed-by: Govind Singh <govinds@qualcomm.corp-partner.google.com>
Commit-Queue: Zhihong Yu <zhihongyu@chromium.org>
Tested-by: Zhihong Yu <zhihongyu@chromium.org>