Revert "ASoC: dapm: mark dapm_kcontrol_get_wlist as global function"

This reverts commit e5c7d7c1.
This change is no longer required as the wrapper available in upstream
snd_soc_dapm_kcontrol_widget() can be used for the same purpose of
accessing the parent widget for a given kcontrol.

Change-Id: I6af4b6cab1f51e3e9321573a08777dbedce62243
Signed-off-by: Banajit Goswami <bgoswami@codeaurora.org>