ASoC: wcd: use wrapper to access parent widget for kcontrol

Use snd_soc_dapm_kcontrol_widget() wrapper available ALSA core
to access the parent widget for a Kcontrol.

Change-Id: I8fd80b59a27ab0b27d0db6ed4dbb9d76db7a42f9
Signed-off-by: Banajit Goswami <bgoswami@codeaurora.org>