PCI: check if bus has a proper bridge device before triggering SBR

For devices attached to the root bus, we can't trigger Secondary Bus
Reset because there is no bridge device associated with the bus. So
need to check bus->self again NULL first before using it.

Reviewed-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: Yu Zhao <yu.zhao@intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>