mfd: twl4030: Fix dummy irq chip usage

The twl irqchip uses the dummy irq chip ack functions, which is NULL
now. Switch it over to use irq_ack.
Reported-and-tested-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

drivers/mfd/twl4030-irq.c

@@ -810,7 +810,7 @@ int twl4030_init_irq(int irq_num, unsigned irq_base, unsigned irq_end)
 	twl4030_irq_chip = dummy_irq_chip;
 	twl4030_irq_chip.name = "twl4030";
 
-	twl4030_sih_irq_chip.ack = dummy_irq_chip.ack;
+	twl4030_sih_irq_chip.irq_ack = dummy_irq_chip.irq_ack;
 
 	for (i = irq_base; i < irq_end; i++) {
 		set_irq_chip_and_handler(i, &twl4030_irq_chip,