[POWERPC] Invert null match behaviour for irq_hosts

Currently if you don't specify a match callback for your irq_host it's
assumed you match everything. This is a kind of opt-out approach, and
turns out to be the exception rather than the rule.

So change the semantics to be opt-in, ie. you don't match anything unless
you provide a match callback.  This in itself isn't very useful, but will
allow us to provide a default match implementation in a subsequent patch.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kernel/irq.c

@@ -523,7 +523,7 @@ struct irq_host *irq_find_host(struct device_node *node)
 	 */
 	spin_lock_irqsave(&irq_big_lock, flags);
 	list_for_each_entry(h, &irq_hosts, link)
-		if (h->ops->match == NULL || h->ops->match(h, node)) {
+		if (h->ops->match != NULL && h->ops->match(h, node)) {
 			found = h;
 			break;
 		}