net/mlx5e: Introduce flow steering API

Move mlx5e_flow_steering struct to fs_en.c to make it private.
Introduce flow_steering API and let other files go through it.

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