wifi: mac80211: check for station first in client probe

When probing a client, first check if we have it, and then
check for the channel context, otherwise you can trigger
the warning there easily by probing when the AP isn't even
started yet. Since a client existing means the AP is also
operating, we can then keep the warning.

Also simplify the moved code a bit.

Reported-by:  <syzbot+999fac712d84878a7379@syzkaller.appspotmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>