ALSA: wm8750: add missing VREF output

Add missing output VREF. After a65f0568
it's critical, since it makes chip routing initialisation to fail.

Signed-off-by: Dmitry Baryshkov <dbaryshkov@gmail.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>