cfg80211: refuse authenticating to same BSSID twice

It is possible that there are different BSS structs with
the same BSSID, but we cannot authenticate with multiple
of them them because we need the BSSID to be unique for
deauthenticating/disassociating.

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