sparc32, sun4d: add comment in empty statement in sun4d_request_irq()

This looked like a bug to me.
Add a comment so next reader is hopefully less confused.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/arch/sparc/kernel/sun4d_irq.c b/arch/sparc/kernel/sun4d_irq.c
index b2e3fbf..609a13c 100644
--- a/arch/sparc/kernel/sun4d_irq.c
+++ b/arch/sparc/kernel/sun4d_irq.c
@@ -307,7 +307,8 @@
 
 	if (action) {
 		if ((action->flags & IRQF_SHARED) && (irqflags & IRQF_SHARED)) {
-			for (tmp = action; tmp->next; tmp = tmp->next);
+			for (tmp = action; tmp->next; tmp = tmp->next)
+				/* find last entry - tmp used below */;
 		} else {
 			ret = -EBUSY;
 			goto out_unlock;