powerpc/pseries: Fix xics build without CONFIG_SMP

desc->affinity doesn't exit in that case. Let's use a macro for
the UP variant of get_irq_server(), it's the easiest way, avoids
evaluating arguments.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/platforms/pseries/xics.c

@@ -191,11 +191,7 @@ static int get_irq_server(unsigned int virq, cpumask_t cpumask,
 	return default_server;
 }
 #else
-static int get_irq_server(unsigned int virq, cpumask_t cpumask,
-			  unsigned int strict_check)
-{
-	return default_server;
-}
+#define get_irq_server(virq, cpumask, strict_check) (default_server)
 #endif
 
 static void xics_unmask_irq(unsigned int virq)