net/mlx5e: Make dma_info array dynamic in struct mlx5e_mpw_info

This commit moves the dma_info array to the end of struct mlx5e_mpw_info
to make it a flexible array. It also removes the intermediate struct
mlx5e_umr_dma_info, which used to contain only this array. The
flexibility of dma_info will allow to choose its size dynamically in a
following commit.

Signed-off-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>