genirq: Fix missing return value in irq_destroy_ipi()

Commit 7cec18a3906b changed the return type of irq_destroy_ipi to int, but
missed adding a value to one return statement. Fix this to silence the
resulting compiler warning:

kernel/irq/ipi.c In function ‘irq_destroy_ipi’:
kernel/irq/ipi.c:128:3: warning: ‘return’ with no value, in function returning non-void [-Wreturn-type]

Fixes: 7cec18a3906b "genirq: Add error code reporting to irq_{reserve,destroy}_ipi"
Signed-off-by: Matt Redfearn <matt.redfearn@imgtec.com>
Cc: linux-mips@linux-mips.org
Link: http://lkml.kernel.org/r/1464086550-24734-1-git-send-email-matt.redfearn@imgtec.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/irq/ipi.c

@@ -125,7 +125,7 @@ int irq_destroy_ipi(unsigned int irq, const struct cpumask *dest)
 
 	domain = data->domain;
 	if (WARN_ON(domain == NULL))
-		return;
+		return -EINVAL;
 
 	if (!irq_domain_is_ipi(domain)) {
 		pr_warn("Trying to destroy a non IPI domain!\n");