mfd: Set twl4030-irq irq nested flag

Threads from twl4030's children will be called
nested in the context of the demultiplexing
handler on twl4030-irq.c.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Reviewed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>