mac80211: don't connect to IBSS network with different privacy

Even when we find an IBSS with the SSID we're looking for, we
may not be able to connect to it because it has a key and we
don't, or vice versa. Avoid such situations by checking the
privacy capability bit.

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