x86: make HAVE_SPARSE_IRQ support selectable

Ingo said sparse_irq is some intrusive. need to make it selectable

to make it simple, remove irq_desc as parameter in some functions.
(ack, eoi, set_affinity).
may need to make member if irq_chip to take irq_desc, or struct irq later.

Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/arch/x86/kernel/irq_32.c b/arch/x86/kernel/irq_32.c
index cc929f2..b2e1082 100644
--- a/arch/x86/kernel/irq_32.c
+++ b/arch/x86/kernel/irq_32.c
@@ -269,7 +269,7 @@
 	struct irqaction * action;
 	unsigned long flags;
 	unsigned int entries;
-	struct irq_desc *desc;
+	struct irq_desc *desc = NULL;
 	int tail = 0;
 
 #ifdef CONFIG_HAVE_SPARSE_IRQ