cfg80211: fix netdev down problem

We shouldn't be looking at the ssid_len for non-IBSS,
and for IBSS we should also return an error on trying
to leave an IBSS while not in or joining an IBSS.

This fixes an issue where we wouldn't disconnect() on
an interface being taken down since there's no SSID
configured this way.

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