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.