PCI: Make pci_bus_add_device() void

pci_bus_add_device() always returns 0, so there's no point in returning
anything at all.  Make it a void function and remove the tests of the
return value from the callers.

[bhelgaas: changelog, remove unused "err" from i82875p_setup_overfl_dev()]
Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>