pci: clean up of pci_set_default_subsystem_id().

Use pci accessor function.
don't return value because it always return 0 and
the caller doesn't check the return value.

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
