net/mlx5e: Fix compatibility issue with ethtool flash device

Cited patch deleted ethtool flash device support, as ethtool core can
fallback into devlink flash callback. However, this is supported only if
there is a devlink port registered over the corresponding netdevice.

As mlx5e do not have devlink port support over native netdevice, it broke
the ability to flash device via ethtool.

This patch re-add the ethtool callback to avoid user functionality breakage
when trying to flash device via ethtool.

Fixes: 9c8bca26 ("mlx5: Move firmware flash implementation to devlink")
Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>