cfg80211: fix wext stats

Instead of using the wext BSSID which may be NULL if
you haven't explicitly set one, we should instead use
the current_bss pointer -- if that's NULL we aren't
connected anyway. Fixes missing signal quality output
reported to me internally at Intel.

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