PCI: Use BAR index in sysfs attr->private instead of resource pointer

We store the pointer, and then on *every* use of it we loop over the
device's resources to find out the index.  That's kind of silly.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>