mfd: Add ab8500 interrupt disable hook

Add the missing interrupt disable hook in the irq_chip
callbacks for ab8500.

Signed-off-by: Virupax Sadashivpetimath <virupax.sadashivpetimath@stericsson.com>
Reviewed-by: Srinidhi Kasagar <srinidhi.kasagar@stericsson.com>
Signed-off-by: Philippe Langlais <philippe.langlais@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

drivers/mfd/ab8500-core.c

@@ -293,6 +293,7 @@ static struct irq_chip ab8500_irq_chip = {
 	.irq_bus_lock		= ab8500_irq_lock,
 	.irq_bus_sync_unlock	= ab8500_irq_sync_unlock,
 	.irq_mask		= ab8500_irq_mask,
+	.irq_disable		= ab8500_irq_mask,
 	.irq_unmask		= ab8500_irq_unmask,
 };