PCI: Remove unused pci_dev_b()

All uses of pci_dev_b() have been replaced by list_for_each_entry(), so
remove it.

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