s390/qeth: cleanup drv attr usage

Cleanup attributes of the qeth ccwgroup driver. Ensure availability
of driver attributes by the time the driver is registered.

Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>