Backport: PCI: allow pci_alloc_child_bus() to handle a NULL bridge

Backport: PCI: allow pci_alloc_child_bus() to handle a NULL bridge

    commit 3789fa8a2e534523c896a32a9f27f78d52ad7d82
    Author: Yu Zhao <yu.zhao@intel.com>
    Date:   Sat Nov 22 02:41:07 2008 +0800

    PCI: allow pci_alloc_child_bus() to handle a NULL bridge

    Allow pci_alloc_child_bus() to allocate buses without bridge
    devices.
    Some SR-IOV devices can occupy more than one bus number, but there
    is no
    explicit bridges because that have internal routing mechanism.

    Signed-off-by: Yu Zhao <yu.zhao@intel.com>
    Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Yu Zhao <yu.zhao@intel.com>