wifi: ath11k: fix memory leak in WMI firmware stats

In the Linux kernel, the following vulnerability has been resolved: 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