mfd: htc-i2cpld: Cleanup interrupt handling

Remove the pointless irq_desc check in set_type. This function is
called with that irq descriptor locked. Also remove the write back of
the flow type as the core code does this already when the return value
is 0.

Also store the flow type in the chip data structure, so there is no
need to fiddle in the irq descriptor.

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