net/mlx5e: Expose function for building RSS params

Towards enabling RSS for the vport representors, extract the
procedure for building a device's RSS params, and expose the
function.

Signed-off-by: Gavi Teitz <gavi@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>