wifi: mt76: mt7921: fix kernel panic due to null pointer dereference

Address a kernel panic caused by a null pointer dereference in the
`mt792x_rx_get_wcid` function. The issue arises because the `deflink` structure
is not properly initialized with the `sta` context. This patch ensures that the
`deflink` structure is correctly linked to the `sta` context, preventing the
null pointer dereference.

 BUG: kernel NULL pointer dereference, address: 0000000000000400
 #PF: supervisor read access in kernel mode
 #PF: error_code(0x0000) - not-present page
 PGD 0 P4D 0
 Oops: Oops: 0000 [#1] PREEMPT SMP NOPTI
 CPU: 0 UID: 0 PID: 470 Comm: mt76-usb-rx phy Not tainted 6.12.13-gentoo-dist #1
 Hardware name:  /AMD HUDSON-M1, BIOS 4.6.4 11/15/2011
 RIP: 0010:mt792x_rx_get_wcid+0x48/0x140 [mt792x_lib]
 RSP: 0018:ffffa147c055fd98 EFLAGS: 00010202
 RAX: 0000000000000000 RBX: ffff8e9ecb652000 RCX: 0000000000000000
 RDX: 0000000000000000 RSI: 0000000000000001 RDI: ffff8e9ecb652000
 RBP: 0000000000000685 R08: ffff8e9ec6570000 R09: 0000000000000000
 R10: ffff8e9ecd2ca000 R11: ffff8e9f22a217c0 R12: 0000000038010119
 R13: 0000000080843801 R14: ffff8e9ec6570000 R15: ffff8e9ecb652000
 FS:  0000000000000000(0000) GS:ffff8e9f22a00000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 0000000000000400 CR3: 000000000d2ea000 CR4: 00000000000006f0
 Call Trace:
  <TASK>
  ? __die_body.cold+0x19/0x27
  ? page_fault_oops+0x15a/0x2f0
  ? search_module_extables+0x19/0x60
  ? search_bpf_extables+0x5f/0x80
  ? exc_page_fault+0x7e/0x180
  ? asm_exc_page_fault+0x26/0x30
  ? mt792x_rx_get_wcid+0x48/0x140 [mt792x_lib]
  mt7921_queue_rx_skb+0x1c6/0xaa0 [mt7921_common]
  mt76u_alloc_queues+0x784/0x810 [mt76_usb]
  ? __pfx___mt76_worker_fn+0x10/0x10 [mt76]
  __mt76_worker_fn+0x4f/0x80 [mt76]
  kthread+0xd2/0x100
  ? __pfx_kthread+0x10/0x10
  ret_from_fork+0x34/0x50
  ? __pfx_kthread+0x10/0x10
  ret_from_fork_asm+0x1a/0x30
  </TASK>
 ---[ end trace 0000000000000000 ]---

Reported-by: Nick Morrow <usbwifi2024@gmail.com>
Closes: morrownr/USB-WiFi#577
Cc: stable@vger.kernel.org
Fixes: 90c1028 ("wifi: mt76: mt7925: Update mt792x_rx_get_wcid for per-link STA")
Signed-off-by: Ming Yen Hsieh <mingyen.hsieh@mediatek.com>
Tested-by: Salah Coronya <salah.coronya@gmail.com>
Link: https://patch.msgid.link/20250218033343.1999648-1-mingyen.hsieh@mediatek.com
Signed-off-by: Felix Fietkau <nbd@nbd.name>

Author: Ming Yen Hsieh

drivers/net/wireless/mediatek/mt76/mt7921/main.c

@@ -811,6 +811,7 @@ int mt7921_mac_sta_add(struct mt76_dev *mdev, struct ieee80211_vif *vif,
 	msta->deflink.wcid.phy_idx = mvif->bss_conf.mt76.band_idx;
 	msta->deflink.wcid.tx_info |= MT_WCID_TX_INFO_SET;
 	msta->deflink.last_txs = jiffies;
+	msta->deflink.sta = msta;
 
 	ret = mt76_connac_pm_wake(&dev->mphy, &dev->pm);
 	if (ret)