mfd: twl6030: Fix endianness problem in IRQ handler

The current TWL 6030 IRQ handler assumes little endianness.
This change makes it endian-neutral.

Signed-off-by: Danke Xie <d.xie@sta.samsung.com>
Signed-off-by: Taras Kondratiuk <taras.kondratiuk@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>