ASoC: dapm: Setup private_free callback for dapm kcontrols

The private data containing the widget list that is a assigned to a DAPM
kcontrol is never freed. Setup the private_free for DAPM kcontrols to take care
of this.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>