wifi: mac80211: drop spurious WARN_ON() in ieee80211_ibss_csa_beacon()

The WARN_ON() in subject was actually seen only once, with 5.10.200
under syzkaller. It looks like a weird artifact of (ab?)using the
syzkaller itself [1], and hopefully may be safely removed.

[1] https://lore.kernel.org/linux-wireless/1bd8f266-dee0-4d4e-9b50-e22546b55763@yandex.ru/T/#uSigned-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Link: https://msgid.link/20231208153130.107409-1-dmantipov@yandex.ruSigned-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/ibss.c

@@ -485,7 +485,7 @@ int ieee80211_ibss_csa_beacon(struct ieee80211_sub_if_data *sdata,
 				ifibss->ssid_len, IEEE80211_BSS_TYPE_IBSS,
 				IEEE80211_PRIVACY(ifibss->privacy));
 
-	if (WARN_ON(!cbss))
+	if (unlikely(!cbss))
 		return -EINVAL;
 
 	rcu_read_lock();