wifi: rtw89: avoid possible TX wait initialization race

The value of skb_data->wait indicates whether skb is passed on to the
core mac80211 stack or released by the driver itself.  Make sure that by
the time skb is added to txwd queue and becomes visible to the completing
side, it has already allocated and initialized TX wait related data (in
case it's needed).

This is found by code review and addresses a possible race scenario
described below:

      Waiting thread                          Completing thread

rtw89_core_send_nullfunc()
  rtw89_core_tx_write_link()
    ...
    rtw89_pci_txwd_submit()
      skb_data->wait = NULL
      /* add skb to the queue */
      skb_queue_tail(&txwd->queue, skb)

  /* another thread (e.g. rtw89_ops_tx) performs TX kick off for the same queue */

                                            rtw89_pci_napi_poll()
                                            ...
                                              rtw89_pci_release_txwd_skb()
                                                /* get skb from the queue */
                                                skb_unlink(skb, &txwd->queue)
                                                rtw89_pci_tx_status()
                                                  rtw89_core_tx_wait_complete()
                                                  /* use incorrect skb_data->wait */
  rtw89_core_tx_kick_off_and_wait()
  /* assign skb_data->wait but too late */

Found by Linux Verification Center (linuxtesting.org).

Fixes: 1ae5ca61 ("wifi: rtw89: add function to wait for completion of TX skbs")
Cc: stable@vger.kernel.org
Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
Acked-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://patch.msgid.link/20250919210852.823912-3-pchelkin@ispras.ru