net/mlx5e: Avoid detaching non-existing netdev under switchdev mode
After introducing dedicated uplink representor, the netdev instance
set over the esw manager vport (PF) became no longer in use, so it was
removed in the cited commit once we're on switchdev mode.
However, the mlx5e_detach function was not updated accordingly, and it
still tries to detach a non-existing netdev, causing a kernel crash.