net/mlx5e: Expose same physical switch_id for all representors

Report system_image_guid as the E-Switch switch_id, this ensures
that when a NIC contains multiple PCI functions and which
has merged eswitch capability, all representors from
multiple PFs publish same switch_id.

Signed-off-by: Paul Blakey <paulb@mellanox.com>
Reviewed-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>