wifi: mac80211: prohibit deactivating all links

In the internal API this calls this is a WARN_ON, but that
should remain since internally we want to know about bugs
that may cause this. Prevent deactivating all links in the
debugfs write directly.

Reported-by:  <syzbot+0c5d8e65f23569a8ffec@syzkaller.appspotmail.com>
Fixes: 3d901102 ("wifi: mac80211: implement link switching")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Link: https://patch.msgid.link/20241230091408.505bd125c35a.Ic3c1f9572b980a952a444cad62b09b9c6721732b@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>