frv/PCI: Use list_for_each_entry() for bus->devices traversal

Replace open-coded list traversal with list_for_each_entry().

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Yinghai Lu <yinghai@kernel.org>
CC: David Howells <dhowells@redhat.com>