s390/qeth: implement ndo_bridge_getlink for learning_sync

Documentation/networking/switchdev.txt and 'man bridge' indicate that the
learning_sync bridge attribute is used to indicate whether a given
device will sync MAC addresses learned on its device port to a master
bridge FDB.

learning_sync attribute can not be read while interface is offline (down).
See
'commit e6e771b3 ("s390/qeth: detach netdevice while card is offline")'
We return EOPNOTSUPP and not EONODEV in this case, because EONOTSUPP is the
only rc that is tolerated by 'bridge -d link show'.

Signed-off-by: Alexandra Winter <wintera@linux.ibm.com>
Reviewed-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>