net/mlx5e: Remove MLX5E_XDP_TX_DS_COUNT

After introducing multi-buffer XDP_TX, the MLX5E_XDP_TX_DS_COUNT define
became misleading. It's no longer the DS count of an XDP_TX WQE, this
WQE can be longer because of fragments.

As this define is only used at one place in mlx5e_open_xdpsq(), it's
also not very useful anymore. This commit removes the define and puts
the calculation of ds_count for prefilled single-fragment WQEs inline.

Signed-off-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>