s390/qdio: clean up q->irq_ptr usage

In tiqdio_call_inq_handlers(), we're looping over all
input queues on the *same* irq. So instead of using the
queues' back pointer, we can just access the irq directly.

No functional change.

Signed-off-by: Julian Wiedmann <jwi@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>