Check to make sure we don't update a BAR with an address we haven't
assigned.

If we haven't assigned an address to a resource, we shouldn't write it to a
BAR.  This isn't a problem for the usual path via pci_assign_resource(),
which clears IORESOURCE_UNSET before calling pci_update_resource(), but
paths like pci_restore_bars() can call this for resources we haven't
assigned.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>