ASoC: dapm: use match_string() helper

match_string() returns the index of an array for a matching string,
which can be used instead of open coded variant.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Liam Girdwood <lgirdwood@gmail.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Jaroslav Kysela <perex@perex.cz>
Cc: Takashi Iwai <tiwai@suse.com>
Cc: alsa-devel@alsa-project.org
Signed-off-by: Yisheng Xie <xieyisheng1@huawei.com>
Signed-off-by: Mark Brown <broonie@kernel.org>