ethtool: Use explicit designated initializers for .cmd

Initialize the .cmd member by using a designated struct
initializer. This fixes warning of missing field initializers,
and makes code a little easier to read.

Signed-off-by: Li RongQing <lirongqing@baidu.com>
Reviewed-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>