ath10k: add cleanup in ath10k_sta_state()

If 'sta->tdls' is false, no cleanup is executed, leading to memory/resource
leaks, e.g., 'arsta->tx_stats'. To fix this issue, perform cleanup before
go to the 'exit' label.

Signed-off-by: Wenwen Wang <wenwen@cs.uga.edu>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>