s390/dasd: Define often used variable

block->request_queue is used many times in dasd_setup_queue. Define a
separate variable to increase readability a bit and to make it better
reusable.

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>