net/mlx5: fix find sibling devices

net/mlx5: fix find sibling devices

The routine mlx5_eth_find_next() and related iterating macro
MLX5_ETH_FOREACH_DEV is used to iterate through sibling devices (all
representors share the same configuration and switching domain) on top
of specified root device.

The root device parameter was specified as NULL, and it caused
missing siblings in iteration during representor device probing,
causing:

1. allocating new domain_id for the device being probed.
2. discrepancy in representor configurations and potential overall
   driver malfunctions.

Fixes: 56bb3c84e982 ("net/mlx5: reduce PCI dependency")

Signed-off-by: Gregory Etelson <getelson@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>