cfg80211: allow cfg80211_connect_result with bssid == NULL

In case of connection failure, the bssid info is not a must have.

Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>