mac80211_hwsim: unregister genetlink family properly

During hwsim_init_netlink(), we should call genl_unregister_family()
if failed on netlink_register_notifier() since the genetlink is
already registered.

Signed-off-by: Su Kang Yin <cantona@cantona.net>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>