also make bus_numa work even if ACPI_NUMA is not defined.

don't call pxm_to_node again, and use node directly.
Signed-off-by: NYinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Signed-off-by: NIngo Molnar <mingo@elte.hu>