pci/bridge: clean up of pci_bridge_initfn()

- use symbolic constant
- use helper function pci_set_xxx()
- removed lines which initializes to 0.
  It is unnecessary because it is already zeroed.
- add some comments on command registers.

Some initial values are suspicious because they seems to
be specific to apb_pci.c which is the only user of pci bridge right now.
For now don't touch those values to avoid breakage.

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
