PCI: Avoid iterating through memory outside the resource window

If the 'image' pointer has been advanced more than 'size', we've already
iterated through memory outside the resource window.

We have zero control over whatever we find in the option ROM, if it's even
an option ROM and not just an accident of random data just happening to
look like an option ROM.

Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>