net/mlx5e: Move struct mlx5e_xmit_data to datapath header

Move TX datapath struct from the generic en.h to the datapath txrx.h
header, where it belongs.

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