ASoC: wm8741: Make function 'wm8741_mute' static

Fixes the following sparse warning:

sound/soc/codecs/wm8741.c:371:5: warning:
 symbol 'wm8741_mute' was not declared. Should it be static?

Fixes: 36b15993 ("ASoC: wm8741: Add digital mute callback")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Mark Brown <broonie@kernel.org>