[S390] dasd: move dasd-diag kmsg to dasd

The DIAG discipline does not have a own driver name. It shows up as
dasd-eckd or dasd-fba. So messages for dasd-diag are moved to the
generic dasd part.

Signed-off-by: Stefan Haberland <stefan.haberland@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>