mac80211: call drv_ibss_join() on restart

If a driver does any significant activity in its ibss_join method,
then it will very well expect that to be called during restart,
before any stations are added. Do that.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>