ASoC: wcd9306: add mixer control to update loopback mode

Digital mute is enabled as part of the DAPM sequence but unmute
is done through digital mute interface on TX path added in the
codec driver which facilitate soc framework to ensure the
decimator path is unmuted when the stream is started. but this
will not holds good for the ADIE loopback usecases.
Add mixer control to update the loopback status and during
the DAPM sequence based on this status unmute the DEC path as part
of DAPM sequence.

Change-Id: Ia2f83841d0c4fde41627e3ec0bce1eeeee1ebe40
Signed-off-by: Aravind Kumar <akumark@codeaurora.org>