wifi: ath12k: fix warning when unbinding

In the Linux kernel, the following vulnerability has been resolved: wifi: ath12k: fix warning when unbinding If there is an error during some initialization related to firmware, the buffers dp->tx_ring[i].tx_status are released. However this is released again when the device is unbinded (ath12k_pci), and we get: WARNING: CPU: 0 PID: 2098 at mm/slub.c:4689 free_large_kmalloc+0x4d/0x80 Call Trace: free_large_kmalloc ath12k_dp_free ath12k_core_deinit ath12k_pci_remove ... The issue is always reproducible from a VM because the MSI addressing initialization is failing. In order to fix the issue, just set the buffers to NULL after releasing in order to avoid the double free.