s390/qeth: simplify L3 sysfs group management

Use the right helpers to create/remove all attribute groups in one go.

Suggested-by: Hans Wippel <hwippel@linux.vnet.ibm.com>
Signed-off-by: Julian Wiedmann <jwi@linux.vnet.ibm.com>
Reviewed-by: Ursula Braun <ubraun@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>