PCI: allow PCI core hotplug to remove PCI root bus

There is no reason to prevent removal of root bus devices. A subsequent
rescan will find them just fine.

Reviewed-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>