wifi: ath12k: fix NULL deref in change_sta_links for unready link

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

wifi: ath12k: fix NULL deref in change_sta_links for unready link

_ieee80211_set_active_links() calls _ieee80211_link_use_channel() for
each newly-added link and WARN_ON_ONCE()s if it fails. The call uses
assign_on_failure=true, which allows mac80211 to continue despite
driver failures, but when a mac80211-level channel validation fails
(e.g., combinations check, DFS, or no available radio),
drv_assign_vif_chanctx() is never reached. Since ath12k_mac_vdev_create()
is only called from that path, arvif->is_created remains false and
arvif->ar remains NULL for the failed link.

The subsequent drv_change_sta_links() call reaches
ath12k_mac_op_change_sta_links(), which allocates an arsta and sets
ahsta->links_map |= BIT(link_id) for the broken link before checking
whether the link is ready. When the vdev was never created, only
station_add() is skipped, but the link remains in links_map.

Any subsequent operation iterating links_map and dereferencing arvif->ar
without a NULL check will crash. Two observed examples are NULL deref in
ath12k_mac_ml_station_remove() on disconnect and in ath12k_mac_op_set_key()
when wpa_supplicant installs PTK keys.

BUG: Unable to handle kernel NULL pointer dereference at 0x00000000
pc : ath12k_mac_station_post_remove+0x40/0xe8 [ath12k]
Call trace:
ath12k_mac_station_post_remove+0x40/0xe8 [ath12k]
ath12k_mac_op_sta_state+0xb60/0x1720 [ath12k]
drv_sta_state+0x100/0xbd8 [mac80211]
__sta_info_destroy_part2+0x148/0x178 [mac80211]
ieee80211_set_disassoc+0x500/0x678 [mac80211]

BUG: Unable to handle kernel NULL pointer dereference at 0x00000000
pc : ath12k_mac_op_set_key+0x1f8/0x2c0 [ath12k]
Call trace:
ath12k_mac_op_set_key+0x1f8/0x2c0 [ath12k]
drv_set_key+0x70/0x100 [mac80211]
ieee80211_key_enable_hw_accel+0x78/0x260 [mac80211]
ieee80211_add_key+0x16c/0x2ac [mac80211]
nl80211_new_key+0x138/0x280 [cfg80211]

Fix this by checking arvif->is_created before calling
ath12k_mac_alloc_assign_link_sta(). This prevents the broken link from
entering links_map, so all subsequent operations iterating the bitmap
are protected. The reliability of arvif->is_created across all error
paths is ensured by the preceding patch.

Tested-on: WCN7850 hw2.0 PCI WLAN.HMT.1.1.c5-00302-QCAHMTSWPL_V1.0_V2.0_SILICONZ-1.115823.3