PCI: Fold stop and remove helpers into their callers

pci_stop_bus_devices() is only two lines of code and is only called by
pci_stop_bus_device(), so I think it's easier to read if we just fold it
into the caller.  Similarly for __pci_remove_behind_bridge().

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