net/mlx5e: Remove unreachable code in mlx5e_xmit()

After some commits, mlx5e_txwqe_build_eseg() lost its ability to return
boolean value and became effectively void.

Change its return type to void and remove unreachable branches.

Signed-off-by: Vladyslav Tarasiuk <vladyslavt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>