misc: pci_endpoint_test: Use pci_irq_vector function

Replace "pdev->irq + index" operation by the pci_irq_vector() call,
that converts from device vector to Linux IRQ.

Suggested-by: Alan Douglas <adouglas@cadence.com>
Signed-off-by: Gustavo Pimentel <gustavo.pimentel@synopsys.com>
[lorenzo.pieralisi@arm.com: updated commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>