ASoC: wm9705: add private structure

Add a private data structure. This is a preparation for a codec which
would need an another data on top of snd_ac97, which will be the case
when an MFD wm97xx device will probe wm9705.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>