net/mlx5e: IPoIB, Use correct timestamp in child receive flow

The current implementation takes the child timestamp object from
the parent since the rq in mlx5i_complete_rx_cqe belongs to the parent.
This change fixes the issue by taking the correct timestamp.

Fixes: 7e7f4780

 ("net/mlx5e: IPoIB, Use hash-table to map between QPN to child netdev")
Signed-off-by: Feras Daoud <ferasda@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>