s390/qeth: Register switchdev event handler

Conditionally register a qeth_l2 switchdev_event handler to handle bridge
to device switchdev events, when at least one qeth interface has the
bridgeport attribute LEARNING_SYNC enabled.

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>