mfd: Leave acknowledgement of WM831x touchscreen IRQs to the driver

The WM831x touchscreen interrupts need acknowledgement even when using
direct signals to the CPU (which don't go through the core) so leave
the acknowledgement up to the touchscreen driver for simplicity.

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