[IA64] Fix pci.c kernel compilation breakage.

The recent change to convert the is_enabled flag in the PCI device to an
atomic count broke the IA64 compilation.

As pcibios_disable_device is only ever called if the reference count
is zero, convert the if to a BUG_ON.

Signed-off-by: Peter Chubb <peterc@gelato.unsw.edu.au>
Signed-off-by: Tony Luck <tony.luck@intel.com>
diff --git a/arch/ia64/pci/pci.c b/arch/ia64/pci/pci.c
index f4edfbf..eb92cef 100644
--- a/arch/ia64/pci/pci.c
+++ b/arch/ia64/pci/pci.c
@@ -564,8 +564,8 @@
 void
 pcibios_disable_device (struct pci_dev *dev)
 {
-	if (dev->is_enabled)
-		acpi_pci_irq_disable(dev);
+	BUG_ON(atomic_read(&dev->enable_cnt));
+	acpi_pci_irq_disable(dev);
 }
 
 void