mfd: Don't allow WM8350 to be built modular

The genirq implementation does not allow modules to implement irq_chips
so the conversion of WM8350 to genirq means we can no longer allow the
driver to be built as a module.

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