ASoC: Use snd_ctl_find_id_mixer() instead of

Merge series from Richard Fitzgerald <rf@opensource.cirrus.com>:

The first two patches change snd_soc_card_get_kcontrol() to use the
core snd_ctl_find_id_mixer() functionality instead of open-coding its
own list walk.

The last patch adds a KUnit test for this, which was tested on the
original and modified code.