ASoC: msm8x16-wcd: fix headset tx record issue

On pmic 2.0, headset record is failing due to
mux setting is not taking effect.
As ADC2 MUX is not mapped to any register from
codec hardware, it needs to be declared as virtual
mux.

Change-Id: Ie236f16dc33b0b8f1e74822e8556a2d5bcf49e88
Signed-off-by: Laxminath Kasam <lkasam@codeaurora.org>