ASoC: wcd934x: Fix headset TX mode setting

Headset TX can be operated in 3 modes, low-power, high-perf
and default. Currently in driver mode is always being set to
default even though mixer controls are set for low-power and
high-perf. Fix the mode setting by correctly parsing the mixer
control value and set it during TX path enable.

CRs-Fixed: 1102584
Change-Id: Idd40a0b471293048833b34dda3ac5044a87fc3c9
Signed-off-by: Phani Kumar Uppalapati <phaniu@codeaurora.org>