wifi: mac80211: check for station first in client probe

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

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.