net/mlx5e: Convert ethtool_steering member of flow_steering struct to pointer

Convert mlx5e_ethtool_steering member of mlx5e_flow_steering to a
pointer, and allocate dynamically for each profile at flow_steering
init.

Signed-off-by: Lama Kayal <lkayal@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>