PCI: Rename local variables to conventional names

"bus" is the conventional name for a "struct pci_bus *" variable.

Tested-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Yinghai Lu <yinghai@kernel.org>