cfg80211: fix connect/disconnect edge cases

commit 51e13359cd5ea34acc62c90627603352956380af upstream.

If we try to connect while already connected/connecting, but
this fails, we set ssid_len=0 but leave current_bss hanging,
leading to errors.

Check all of this better, first of all ensuring that we can't
try to connect to a different SSID while connected/ing; ensure
that prev_bssid is set for re-association attempts even in the
case of the driver supporting the connect() method, and don't
reset ssid_len in the failure cases.

While at it, also reset ssid_len while disconnecting unless we
were connected and expect a disconnected event, and warn on a
successful connection without ssid_len being set.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>