PCI/ACPI: Use acpi_driver_data() rather than searching acpi_pci_roots

This patch changes the implementation of acpi_pci_find_root().

We can access acpi_pci_root without scanning acpi_pci_roots list.
If hostbridge hotplug is supported, acpi_pci_roots list will be
protected by mutex.  We should not access acpi_pci_roots list
if preventable to lessen deadlock risk.

Signed-off-by: Taku Izumi <izumi.taku@jp.fujitsu.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>