mfd: Set twl4030-irq tertiary interrupts to be nested/threaded.

As tertiary interrupts are handled by handle_twl4030_sih calling
handle_nested_irq, they do not need their own separate irq thread.
So mark them as 'nested_thread' interrupts to avoid the extra thread
creation.

Tested on GTA04 Pheonux.

Signed-off-by: NeilBrown <neilb@suse.de>
Tested-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>