ethtool: Fix link extended state for big endian

The link extended sub-states are assigned as enum that is an integer
size but read from a union as u8, this is working for small values on
little endian systems but for big endian this always give 0. Fix the
variable in the union to match the enum size.

Fixes: ecc31c60 ("ethtool: Add link extended state")
Signed-off-by: Moshe Tal <moshet@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Tested-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>