ACPI / PCI root: Clear driver_data before failing enumeration

If a PCI host bridge cannot be enumerated due to an error in
pci_acpi_scan_root(), its ACPI device object's driver_data field
has to be cleared by acpi_pci_root_add() before freeing the
object pointed to by that field, or some later acpi_pci_find_root()
checks that should fail may succeed and cause quite a bit of
confusion to ensue.

Fix acpi_pci_root_add() to clear device->driver_data before
returning an error code as appropriate.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Toshi Kani <toshi.kani@hp.com>