asoc: codecs: wcd938x: update tx mode setting

Change tx mode setting from dapm widget to kcontrol
as it is not needed in codec route and only needed
for updating mode value.

Change-Id: I5d66b9a82b541b97f5c5789fe2e49d329c6952a8
Signed-off-by: Karthikeyan Mani <kmani@codeaurora.org>