When hot-plugging a root bridge, we always prevent assigning a bus number
that already exists. This makes sure we don't step over an existing bus.
But sometimes we only remove PCI device in PCI hieratchy of OS, i,e.

echo 1 > /sys/bus/pci/devices/.../remove

but actually don't hotplug this device out the platform, so in this case
we still should re-scan this bus to enumerate this device when re-scanning
PCI again.
Signed-off-by: NTiejun Chen <tiejun.chen@windriver.com>
Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>