net/mlx5e: Rename xmit-related structs to generalize them

As preparation for the upcoming TX MPWQE support for SKBs, rename struct
mlx5e_xdp_mpwqe to mlx5e_tx_mpwqe and move it above struct mlx5e_txqsq.
This structure will be reused in the regular SQ and in the regular TX
data path. Also rename mlx5e_xdp_xmit_data to mlx5e_xmit_data - it will
be used in the upcoming TX MPWQE flow.

Signed-off-by: Maxim Mikityanskiy <maximmi@mellanox.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>