Input: wm831x-on - convert to use genirq

Now that the WM831x core has been converted to use genirq for the
interrupt controller there is no need for the client drivers to
use a WM831x-specific API rather than just calling genirq directly.

Also fixes a leak of the IRQ during init failure - the error path
free_irq() was using NULL rather than the driver data as the data
pointer so free_irq() wouldn't have matched.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>