ethtool: Use explicit designated initializers for .cmd

Initialize the .cmd member of various ethtool using a designated struct
initializer rather.  This makes things a teeny bit more robust, although
the chance of a struct layout changing is extremely remote, and also
makes the code a little easier to read.

Signed-off-by: Roland Dreier <rolandd@cisco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>