cfg80211: allow aborting in-progress connection atttempts

On a disconnect request from userspace, cfg80211 currently calls
called rdev_disconnect() only in case that 'current_bss' was set,
i.e. connection had been established.

Change this to allow the userspace call to succeed and call the
driver's disconnect() method also while the connection attempt is
in progress, to be able to abort attempts.

Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
[change commit subject/message]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>