cfg80211: wext: don't display BSSID unless associated

Currently, cfg80211's SIOCGIWAP implementation returns
the BSSID that the user set, even if the connection has
since been dropped due to other changes. It only should
return the current BSSID when actually connected.

Also do a small code cleanup.

Reported-by: Thomas H. Guenther <thomas.h.guenther@intel.com>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Tested-by: Thomas H. Guenther <thomas.h.guenther@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>