PCI: Move pci_map_rom() error path

Move pci_map_rom() error code to the end to prepare for adding another
error path.  No functional change intended.

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>