ath10k: free collected fw stats memory if .pull_fw_stats fails

If .pull_fw_stats() fails for some reason while processing
fw stats event, collected pdev/vdev/peer stats just before
the failure should be freed. This is unlikely to happen,
just code review catch.

Signed-off-by: Raja Mani <rmani@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>