ethtool: add default notification handler

The ethtool netlink notifications have the same format as related GET
replies so that if generic GET handling framework is used to process GET
requests, its callbacks and instance of struct get_request_ops can be
also used to compose corresponding notification message.

Provide function ethnl_std_notify() to be used as notification handler in
ethnl_notify_handlers table.

Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>