ath10k: fixup extended per sta tx statistics

I had a couple concerns with this code.  First, we definitely need to
set the error code if the kzalloc() fails.  Secondly, I was worried that
if we didn't set "arsta->tx_stats" to NULL after freeing it, then it
looks to me like it might lead to a use after free.  I can't test that,
but it's harmless to set it to NULL so I did.

Fixes: a904417f ("ath10k: add extended per sta tx statistics support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>