pci_bridge_write_config() was not being used.

Cc: qemu-stable@nongnu.org
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Signed-off-by: NAndreas Färber <afaerber@suse.de>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>