net/mlx5e: add op for getting netdev DMA device

net/mlx5e: add op for getting netdev DMA device

For zero-copy (devmem, io_uring), the netdev DMA device used
is the parent device of the net device. However that is not
always accurate for mlx5 devices:
- SFs: The parent device is an auxdev.
- Multi-PF netdevs: The DMA device should be determined by
  the queue.

This change implements the DMA device queue API that returns the DMA
device appropriately for all cases.

Signed-off-by: Dragos Tatulea <dtatulea@nvidia.com>
Reviewed-by: Mina Almasry <almasrymina@google.com>
Link: https://patch.msgid.link/20250827144017.1529208-6-dtatulea@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>