MFD: twl6040: Remove wrapper for threaded irq request

Remove the twl6040_request_irq/free_irq inline functions,
and use direct calls instead in the core driver to
register the threaded irq.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Reviewed-by: Felipe Balbi <balbi@ti.com>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>