ethtool: mark netlink family as __ro_after_init

ethtool: mark netlink family as __ro_after_init

Like all genl families ethtool_genl_family needs to not
be a straight up constant, because it's modified/initialized
by genl_register_family(). After init, however, it's only
passed to genlmsg_put() & co. therefore we can mark it
as __ro_after_init.

Since genl_family structure contains function pointers
mark this as a fix.

Fixes: 7dccc0028581 ("ethtool: introduce ethtool netlink interface")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>