powerpc/pmac: Fix SMP kernels on pre-core99 UP machines

The code for "powersurge" SMP would kick in and cause a crash
at boot due to the lack of a NULL test.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/platforms/powermac/smp.c

@@ -414,7 +414,7 @@ static struct irqaction psurge_irqaction = {
 
 static void __init smp_psurge_setup_cpu(int cpu_nr)
 {
-	if (cpu_nr != 0)
+	if (cpu_nr != 0 || !psurge_start)
 		return;
 
 	/* reset the entry point so if we get another intr we won't