mfd: wm831x-spi: Add NULL check before pointer dereference

Add NULL check before dereferencing pointer of_id in order to avoid
a potential NULL pointer dereference.

Addresses-Coverity-ID: 1408830
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>