ethtool: set device private flags with PRIVFLAGS_SET request

Implement PRIVFLAGS_SET netlink request to set private flags of a network
device. These are traditionally set with ETHTOOL_SPFLAGS ioctl request.

Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>