s390/qeth: handle sysfs error during initialization

When setting up the device from within the layer discipline's
probe routine, creating the layer-specific sysfs attributes can fail.
Report this error back to the caller, and handle it by
releasing the layer discipline.

Signed-off-by: Ursula Braun <ubraun@linux.vnet.ibm.com>
[jwi: updated commit msg, moved an OSN change to a subsequent patch]
Signed-off-by: Julian Wiedmann <jwi@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>