mfd: max77686: Return correct error when pdata isn't found

When platform data is not found an -EIO (I/O error) code is returned.
This doesn't seem to be the correct error so better return -EINVAL
(Invalid argument) which is what most drivers do in this case.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Lee Jones <lee.jones@linaro.org>