sgi-agp: Use list_for_each_entry() for bus->devices traversal

Replace list_for_each() + pci_dev_b() with the simpler
list_for_each_entry().

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Yinghai Lu <yinghai@kernel.org>
CC: David Airlie <airlied@linux.ie>