net/mlx5e: IPoIB, Fix child PKEY interface stats on rx path

The current code always does the accounting using the
stats from the parent interface (linked in the rq). This
doesn't work when there are child interfaces configured.

Fix this behavior by always using the stats from the child
interface priv. This will also work for parent only
interfaces: the child (netdev) and parent netdev (rq->netdev)
will point to the same thing.

Fixes: be98737a ("net/mlx5e: Use dynamic per-channel allocations in stats")
Signed-off-by: Dragos Tatulea <dtatulea@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>