wifi: ath6kl: reduce WARN to dev_dbg() in callback

The warn is triggered on a known race condition, documented in the code above
the test, that is correctly handled.  Using WARN() hinders automated testing.
Reducing severity.

Fixes: de2070fc ("ath6kl: Fix kernel panic on continuous driver load/unload")
Reported-and-tested-by: syzbot+555908813b2ea35dae9a@syzkaller.appspotmail.com
Signed-off-by: Oliver Neukum <oneukum@suse.com>
Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20230126182431.867984-1-pchelkin@ispras.ru

drivers/net/wireless/ath/ath6kl/htc_pipe.c

@@ -960,8 +960,8 @@ static int ath6kl_htc_pipe_rx_complete(struct ath6kl *ar, struct sk_buff *skb,
 	 * Thus the possibility of ar->htc_target being NULL
 	 * via ath6kl_recv_complete -> ath6kl_usb_io_comp_work.
 	 */
-	if (WARN_ON_ONCE(!target)) {
-		ath6kl_err("Target not yet initialized\n");
+	if (!target) {
+		ath6kl_dbg(ATH6KL_DBG_HTC, "Target not yet initialized\n");
 		status = -EINVAL;
 		goto free_skb;
 	}