wifi: ath11k: fix memory leak in WMI firmware stats

Memory allocated for firmware pdev, vdev and beacon statistics
are not released during rmmod.

Fix it by calling ath11k_fw_stats_free() function before hardware
unregister.

While at it, avoid calling ath11k_fw_stats_free() while processing
the firmware stats received in the WMI event because the local list
is getting spliced and reinitialised and hence there are no elements
in the list after splicing.

Tested-on: QCN9074 hw1.0 PCI WLAN.HK.2.7.0.1-01744-QCAHKSWPL_SILICONZ-1

Signed-off-by: P Praneesh <quic_ppranees@quicinc.com>
Signed-off-by: Aditya Kumar Singh <quic_adisi@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20230606091128.14202-1-quic_adisi@quicinc.com