cfg80211: don't look at wdev->ssid for giwessid

This variable is only used internally, _while_ connected.
If we use it, the sequence

# iwconfig wlan1 essid foo
<connects>
# iwconfig wlan1 essid ""
<disconnects>
# iwconfig

will still display "foo" as the SSID afterwards, which
is obviously quite bogus. Fix this by only displaying
the wext SSID, if present.

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