wifi: mac80211: don't WARN for connections on invalid channels

It's not clear (to me) how exactly syzbot managed to hit this,
but it seems conceivable that e.g. regulatory changed and has
disabled a channel between scanning (channel is checked to be
usable by cfg80211_get_ies_channel_number) and connecting on
the channel later.

With one scenario that isn't covered elsewhere described above,
the warning isn't good, replace it with a (more informative)
error message.

Reported-by: [e-mail]
Link: https://patch.msgid.link/20251202102511.5a8fb5184fa3.I961ee41b8f10538a54b8565dbf03ec1696e80e03@changeid
Signed-off-by: Johannes Berg <[e-mail]>

net/mac80211/mlme.c

···
 
 	while (!ieee80211_chandef_usable(sdata, &chanreq->oper,
 					 IEEE80211_CHAN_DISABLED)) {
-		if (WARN_ON(chanreq->oper.width == NL80211_CHAN_WIDTH_20_NOHT)) {
+		if (chanreq->oper.width == NL80211_CHAN_WIDTH_20_NOHT) {
+			link_id_info(sdata, link_id,
+				     "unusable channel (%d MHz) for connection\n",
+				     chanreq->oper.chan->center_freq);
 			ret = -EINVAL;
 			goto free;
 		}