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>