[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: NPeter Chubb <peterc@gelato.unsw.edu.au>
Signed-off-by: NTony Luck <tony.luck@intel.com>

arch/ia64/pci/pci.c

@@ -564,7 +564,7 @@ pcibios_enable_device (struct pci_dev *dev, int mask)
 void
 pcibios_disable_device (struct pci_dev *dev)
 {
-	if (dev->is_enabled)
+	BUG_ON(atomic_read(&dev->enable_cnt));
 	acpi_pci_irq_disable(dev);
 }