s390/qeth: stop using struct ccwgroup driver for discipline callbacks

The interface between qeth and its disciplines should not depend on
struct ccwgroup_driver, as a qeth discipline is not a ccwgroup
driver on its own. Instead provide the particular callbacks in struct
qeth_discipline.

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>