cfg80211: re-join IBSS when privacy changes

When going from/to a WEP protected IBSS, we need to
leave this one and join a new one to take care of
the changed capability.

Cc: Hong Zhang <henryzhang62@yahoo.com>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>