cfg80211: fix nl80211 disconnected events

When reporting a disconnection to userspace, we try
to report whether it was from the AP or by our own
choice. However, we misreported a broadcast deauth
or disassoc as being by own choice, which is wrong.
Fix this by checking the sender address instead of
the destination address.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>