s390/dasd: Replace simple_strtoul with kstrtouint

simple_strtoul() has been marked obsolete for quite some time now.
Replace a few last occurrences with kstrtouint().

Reviewed-by: Stefan Haberland <sth@linux.vnet.ibm.com>
Signed-off-by: Jan Höppner <hoeppner@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>