ASoC: wm5102: Initialize dac_comp_lock mutex

Commit d74bcaae

 (ASoC: wm5102: Move ultrasonic response settings
lock to the driver level) created a driver local mutex for protecting
the ultrasonic response settings but neglected to initialize that mutex,
causing loud complaints from lockep and potential runtime failures. Fix
this by initializing the mutex.

Signed-off-by: Mark Brown <broonie@kernel.org>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>