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.