s390/dasd: Make module parameter visible in sysfs

Change the visibility of the dasd parameter of kernel module dasd_mod
to be consistent with the eer_pages parameter.

Signed-off-by: Peter Oberparleiter <oberpar@linux.vnet.ibm.com>
Acked-by: Stefan Weinhuber <wein@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>