ACPI: pci_root: use driver data rather than list lookup

There's no need to search the list to find the acpi_pci_root
structure.  We saved it as device->driver_data when we added
the device.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Len Brown <len.brown@intel.com>