net/switchdev: Export the same parent ID service function

This helper serves to know if two switchdev port netdevices belong to the
same HW ASIC, e.g to figure out if forwarding offload is possible between them.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>