s390/qdio: simplify thinint device registration

On an interrupt, tiqdio_thinint_handler() walks a list of all objects
that might require attention, and checks their DSCI. This list is
awkwardly built from Input Queues, even though the IRQs are per-device
and the queue is then only used to dereference its qdio_irq parent.

To simplify the logic, change the code so that tiq_list contains
qdio_irq entries.

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>