mfd: Fix wm8994_device_init() return value

wm8994_device_init() will return 0 in the case of kzalloc fail
in current implementation.
This patch fixes the return value.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>