PCI: frv: replace now removed pci_bus_b() causing build failure

In commit 94e6a9b93064 ("PCI: Remove pci_bus_b() and use
list_for_each_entry() directly") the function pci_bus_b() was removed, but
one instance of its usage was missed.

Replace it in the same fashion that the original commit did.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Yijing Wang <wangyijing@huawei.com>