x86, pci: Add sanity check for PCI fixed bar probing

While probing for the PCI fixed BAR capability in the extended PCI
configuration space we need to make sure raw_pci_ext_ops is
actually initialized.

Signed-off-by: Jacob Pan <[e-mail]>
LKML-Reference: <[e-mail]>
Signed-off-by: H. Peter Anvin <[e-mail]>

arch/x86/pci/mrst.c

···
 	u32 pcie_cap = 0, cap_data;
 
 	pos = PCIE_CAP_OFFSET;
+
+	if (!raw_pci_ext_ops)
+		return 0;
+
 	while (pos) {
 		if (raw_pci_ext_ops->read(pci_domain_nr(bus), bus->number,
 					  devfn, pos, 4, &pcie_cap))