net/mlx5e: Reorganize struct mlx5e_rq

Bring fast-path fields together, and combine RX WQE mutual
exclusive fields into a union.

Page-reuse and XDP are mutually exclusive and cannot be used at
the same time.
Use a union to combine their footprints.

Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>