wifi: ath12k: fix NULL pointer dereference in rhash table destroy

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

wifi: ath12k: fix NULL pointer dereference in rhash table destroy

When unbinding the ath12k driver, kernel NULL pointer dereferences
occur in irq_work_sync() called from rhashtable_destroy().

Two hash tables are affected:
1. ath12k_link_sta hash table in ath12k_base
2. ath12k_dp_link_peer hash table in ath12k_dp

The issue happens because the destroy functions are called unconditionally
in cleanup paths, but the hash tables are only initialized late in their
respective init functions. If the device was never fully started or if the
init functions failed before initializing the hash tables, the pointers
will be NULL. The issues are always reproducible from a VM because the MSI
addressing initialization is failing.

Call trace for ath12k_link_sta_rhash_tbl_destroy:
RIP: irq_work_sync+0x1e/0x70
rhashtable_destroy+0x12/0x60
ath12k_link_sta_rhash_tbl_destroy+0x19/0x40 [ath12k]
ath12k_core_stop+0xe/0x80 [ath12k]
ath12k_core_hw_group_cleanup+0x6b/0xb0 [ath12k]
ath12k_pci_remove+0x60/0x110 [ath12k]

Call trace for ath12k_dp_link_peer_rhash_tbl_destroy:
RIP: irq_work_sync+0x1e/0x70
rhashtable_destroy+0x12/0x60
ath12k_dp_link_peer_rhash_tbl_destroy+0x29/0x50 [ath12k]
ath12k_dp_cmn_device_deinit+0x21/0x140 [ath12k]
ath12k_core_hw_group_cleanup+0x6b/0xb0 [ath12k]
ath12k_pci_remove+0x60/0x110 [ath12k]

Fix this by adding NULL checks before calling rhashtable_destroy() in
both destroy functions.

The NULL check approach was chosen because the rhashtable pointer
serves as the initialization state indicator. The init can fail at
various points, leaving some components uninitialized. Checking the
pointer directly is simpler than adding separate state flags that
would need synchronization.