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: NSam Ravnborg <sam@ravnborg.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

arch/sparc/kernel/sun4d_irq.c

@@ -307,7 +307,8 @@ int sun4d_request_irq(unsigned int irq,
 
 	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;