s390/qeth: Update MACs of LEARNING_SYNC device

Update the MAC addresses that are registered with a LEARNING_SYNC qeth
device with the events announced by the attached software bridge.

Typically the LEARNING_SYNC qeth bridge port has an isolated sibling (the
default interface of an 'HiperSockets Converged Interface' (HSCI)). Update
the MACs of isolated siblings as well, to avoid unnecessary flooding in
the attached virtualized switches.

Signed-off-by: Alexandra Winter <wintera@linux.ibm.com>
Reviewed-by: Wenjia Zhang <wenjia@linux.ibm.com>
Signed-off-by: Karsten Graul <kgraul@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>