irqdomain: Fix irq_create_direct_mapping() to test irq_domain type.

irq_create_direct_mapping can only be used with the NOMAP type.  Make
the function test to ensure it is passed the correct type of
irq_domain.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Rob Herring <rob.herring@calxeda.com>

kernel/irq/irqdomain.c

@@ -481,8 +481,8 @@ unsigned int irq_create_direct_mapping(struct irq_domain *domain)
 	if (domain == NULL)
 		domain = irq_default_domain;
 
-	BUG_ON(domain == NULL);
-	WARN_ON(domain->revmap_type != IRQ_DOMAIN_MAP_NOMAP);
+	if (WARN_ON(!domain || domain->revmap_type != IRQ_DOMAIN_MAP_NOMAP))
+		return 0;
 
 	virq = irq_alloc_desc_from(1, of_node_to_nid(domain->of_node));
 	if (!virq) {