s390/qdio: set qdio_irq->cdev at allocation time

Set up qdio_irq->cdev right when the qdio_irq struct is allocated, so
that all subsequent code can rely on this pointer.

Then convert two helper functions to not pass a cdev parameter around.

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