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

In the Linux kernel, the following vulnerability has been resolved:

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.