ASoC: dapm: Add snd_soc_dapm_kcontrol_widget()

Given a kcontrol, we may want to access the parent widget
and it's associated data. So export function to return it.

Signed-off-by: Mythri P K <mythri.p.k@intel.com>
Signed-off-by: Jeeja KP <jeeja.kp@intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>