rtlwifi: Use pr_warn instead of WARN_ONCE

This memory allocation failure can be triggered by fault injection or
high pressure testing, resulting a WARN.

Fix this by replacing WARN with pr_warn.
Reported-by: syzkaller <syzkaller@googlegroups.com>
Signed-off-by: Dongliang Mu <mudongliangabcd@gmail.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220511014453.1621366-1-dzm91@hust.edu.cn

drivers/net/wireless/realtek/rtlwifi/usb.c

@@ -1014,7 +1014,7 @@ int rtl_usb_probe(struct usb_interface *intf,
 	hw = ieee80211_alloc_hw(sizeof(struct rtl_priv) +
 				sizeof(struct rtl_usb_priv), &rtl_ops);
 	if (!hw) {
-		WARN_ONCE(true, "rtl_usb: ieee80211 alloc failed\n");
+		pr_warn("rtl_usb: ieee80211 alloc failed\n");
 		return -ENOMEM;
 	}
 	rtlpriv = hw->priv;