mfd: twl6030: Cleanup interrupt handling

irq_desc checking in the interrupt demux routine is totally
pointless. The driver sets those lines up, so that cant go away
magically.

Remove the open coded handler magic and use the proper accessor.

This driver needs to be converted to threaded interrupts and buslock.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>