net/mlx5e: Move XDP struct and enum to XDP header

Move struct mlx5e_xdp_info and enum mlx5e_xdp_xmit_mode from the generic
en.h to the XDP header, where they belong.

Reviewed-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>