ASoC: wm8750: Make VREF a VMID instead of OUTPUT widget

Commit 04489eeb ("ALSA: wm8750: add missing VREF output") added the
previously missing VREF widget. And while adding the widget as OUTPUT widget
silences the error of the missing widget it should have rather been a VMID
widget.

The VREF widget is only used internally as a source for the VREF input of
the Out3 Mux. Selecting that input means that the path should be kept
powered up at the VREF/VMID voltage, but that there is no actual audio
signal. Using a VMID widget will do this, whereas using a OUTPUT widget will
simply cause the path to completely power down.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>