mfd: wm831x: Fix possible NULL pointer dereference

The driver always checks for pdata being NULL except in one place.
Add a check to prevent a possible NULL pointer deference error.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>