ASoC: mediatek: mt8188: Simplify with snd_ctl_find_id_mixer()

Replace an open code with the new snd_ctl_find_id_mixer().
There is no functional change.

Acked-by: Mark Brown <broonie@kernel.org>
Cc: Trevor Wu <trevor.wu@mediatek.com>
Link: https://lore.kernel.org/r/20230720082108.31346-11-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>