wifi: cfg80211: ocb: don't leave if not joined

If there's no OCB state, don't ask the driver/mac80211 to
leave, since that's just confusing. Since set/clear the
chandef state, that's a simple check.

Reported-by:  <syzbot+09d1cd2f71e6dd3bfd2c@syzkaller.appspotmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>