mfd: Add __devinit and __devexit annotations in wm8994

The wm8994_device_init() and wm8994_device_exit() functions were not
annotated as device init and exit functions, meaning they shouldn't
reference __devinitdata.

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