ethdev: move RSS expansion code to mlx5 driver

ethdev: move RSS expansion code to mlx5 driver

Patch [1] added support for RSS flow expansion.
It was added in ethdev for public use, but until now it is used only
by MLX5 PMD.
To allow local changes in this code, this patch removes it from ethdev
and moves it to MLX5 PMD file.

[1] commit 4ed05fcd441b ("ethdev: add flow API to expand RSS flows")

Signed-off-by: Dekel Peled <dekelp@nvidia.com>
Acked-by: Matan Azrad <matan@nvidia.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>