mfd: Ensure WM831x charger interrupts are acknowledged when suspending

The charger interrupts on the WM831x are unconditionally a wake source
for the system. If the power driver is not able to monitor them (for
example, due to the IRQ line not having been wired up on the system)
then any charger interrupt will prevent the system suspending for any
meaningful amount of time since nothing will ack them.

Avoid this issue by manually acknowledging these interrupts when we
suspend the WM831x core device if they are masked. If software is
actually using the interrupts then they will be unmasked and this
change will have no effect.

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