ACPI: pci: Do not clear pci_dev->irq in acpi_pci_irq_disable()

ACPI: pci: Do not clear pci_dev->irq in acpi_pci_irq_disable()

Xen pciback driver assumes that pci_dev->irq won't change after calling
pci_disable_device(). But commit cffe0a2b5a34c95a4dadc9ec7132690a5b0f6687
("x86, irq: Keep balance of IOAPIC pin reference count") frees irq
resources and resets pci_dev->irq to zero when pci_disable_device() is
called.

So this is a hotfix for 3.19 to avoid resetting pci_dev->irq, and
another proper fix will be prepared for next merging window.

Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Tested-by: Sander Eikelenboom <linux@eikelenboom.it>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: David Vrabel <david.vrabel@citrix.com>
Cc: Rafael J. Wysocki <rjw@rjwysocki.net>
Cc: Len Brown <lenb@kernel.org>
Link: http://lkml.kernel.org/r/1421720467-7709-3-git-send-email-jiang.liu@linux.intel.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>