net/mlx5e: Simplify querying port representor parent id

System image GUID doesn't depend on eswitch switchdev mode.

Hence, remove the check which simplifies the code.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Vu Pham <vuhuong@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>