firewire: ohci: use pci_irq_vector() to retrieve allocated interrupt line

The pci_irq_vector() is available to retrieve the allocated interrupt line
instead of the direct access to the member of device structure.

Link: https://lore.kernel.org/r/20240331135037.191479-5-o-takashi@sakamocchi.jp


Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>