s390/claw: cleanup drv attr usage

Rename attributes of the claw_group_driver. Also get rid of
the call to driver_remove_file.

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>