ath10k: fix memory leak of tpc_stats

Currently tpc_stats is allocated and is leaked on the return
path if num_tx_chain is greater than WMI_TPC_TX_N_CHAIN. Avoid
this leak by performing the check on num_tx_chain before the
allocation of tpc_stats.

Detected by CoverityScan, CID#1469422 ("Resource Leak")
Fixes: 4b190675 ("ath10k: fix kernel panic while reading tpc_stats")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>