ath10k: fix memory leak for tpc_stats_final

The memory of ar->debug.tpc_stats_final is reallocated every debugfs
reading, it should be freed in ath10k_debug_destroy() for the last
allocation.

Tested HW: QCA9984
Tested FW: 10.4-3.9.0.2-00035

Signed-off-by: Miaoqing Pan <miaoqing@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>