s390/dasd: simplify locking in process_final_queue

Simplify locking in __dasd_device_process_final_queue to fix
the following sparse warning:

drivers/s390/block/dasd.c:1902:9: warning:
context imbalance in '__dasd_device_process_final_queue' - different lock contexts for basic block

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