[S390] dcssblk: revert devt conversion

git commit f331c029 changed users of
->first_minor to devt. This broke device handling in dcssblk, so that
no additional devices could be added after the first one.

This patch reverts the devt conversion to the previous ->first_minor
handling.

Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>