s390/qdio: clean up cdev access in qdio_setup_irq()

Some parts use init_data->cdev, others use irq_ptr->cdev. In the end
it's all the same, but unnecessarily confusing.

Use a single reference instead.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Reviewed-by: Steffen Maier <maier@linux.ibm.com>
Reviewed-by: Benjamin Block <bblock@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>