net/mlx5e: Report header-data split state through ethtool

HW-GRO (SHAMPO) packet merger scheme implies header-data split in the
driver, report it through the ethtool interface.

Signed-off-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>