mac80211: send deauth only with channel context

When userspace asks to deauthenticate and we're just
authenticated (or still authenticating) send a deauth
frame instead of deleting the auth request.

On the other hand, if we've just disassociated and
therefore deleted all our state already, drop the
deauth request because we no longer have a channel
context to send it on.

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