net: ethtool: Make helpers public

Make some helpers for building ethtool netlink messages available
outside the compilation unit, so they can be used for building
messages which are not simple get/set.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>