mfd: Acknowlege all WM831x IRQs before we handle them

Ensure that we never have a window where we've handled an interrupt (and
therefore need to be notified of new events) but haven't yet told the
interrupt controller that this is the case (so any new events will be
discarded).

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