mfd: wm97xx-core: Platform data can be NULL

It is not mandatory that platform data is passed along the ac97
codec. Actually there are configuration without a battery connected to
the ADC of the codec.

This is for example the case for the PXA zylonite platform, so fix the
NULL dereferencing by adding a test.

Fixes: a5c6951c

 ("mfd: wm97xx-core: core support for wm97xx Codec")
Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Lee Jones <lee.jones@linaro.org>