mfd: max899x: Avoid redundant irq_data lookup

It's pretty silly to do

     irq_data *d = irq_get_irq_data(irq_data->irq);

because that results in d = irq_data, but goes through a lookup of the
irq_data. Use irq_data directly.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>