mfd: Don't allow wm831x to be built as a module

The genirq infrastructure functions aren't currently exported,
preventing modular builds.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>