PCI: Fail pci_map_rom() if the option ROM is invalid

If we detect a invalid PCI option ROM (e.g., invalid ROM header signature),
we should unmap it immediately and fail. It doesn't make any sense to
return a mapped area with size of 0.

I have seen this case on Intel GVTg vGPU, which has no VBIOS. It will not
cause a real problem, but we should skip it as early as possible.

Signed-off-by: Changbin Du <changbin.du@intel.com>
[bhelgaas: split non-functional change into separate patch]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>