[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>