cfg80211: clear WEXT SSID when clearing IBSS

When we leave an IBSS, we should clear the SSID and not just the
BSSID, but since WEXT allows configuring while the interface is
down we must not clear it when leaving due to taking the iface
down, so some complications are needed.

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