s390/qeth: clean up device_type management

qeth uses three device_type structs - a generic one, and one for each
sub-driver (which is used for fixed-layer devices only). Instead of
exporting these device_types back&forth between the driver's modules,
make all the logic self-contained within the sub-drivers.

On disc->setup() they either install their own device_type, or add the
sysfs attributes that are missing in the generic device_type. Later on
disc->remove() these attributes are removed again from any device that
has the generic device_type.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>