PCI/portdrv: Remove warning about invalid IRQ for hot-added PCIe ports

For hot-added PCIe ports on x86 platforms, we always warned about an
invalid IRQ, e.g.,

  pci 0000:00:00.0: device [8086:0e0b] has invalid IRQ; check vendor BIOS

This was because we check pci_dev->irq before actually allocating the IRQ
for the device, which happens in this path:

  pcie_port_device_register
    pci_enable_device
      pci_enable_device_flags
        do_pci_enable_device
          pcibios_enable_device    (on x86)
            pcibios_enable_irq

This warning message isn't generated for PCIe ports present at boot time
because x86 arch code has called acpi_pci_irq_enable() in pci_acpi_init()
for each PCI device for safety.

[bhelgaas: changelog]
Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>