s390/dasd: fix hanging offline processing due to canceled worker

During offline processing two worker threads are canceled without
freeing the device reference which leads to a hanging offline process.

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