PCI: designware: Drop pci_fixup_irqs()

Since, through struct pci_host_bridge.map/swizzle_irq hooks, IRQs are now
allocated in the pci_assign_irq() callback automatically, PCI host bridge
drivers can stop relying on pci_fixup_irqs() for IRQ allocation.

Drop pci_fixup_irqs() usage from PCI designware host bridge driver.

Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Jingoo Han <jingoohan1@gmail.com>
Cc: Joao Pinto <Joao.Pinto@synopsys.com>