ASoC: atmel: tse850: Make some functions static

Fix sparse warnings:

sound/soc/atmel/tse850-pcm5142.c:120:5: warning: symbol 'tse850_get_mix' was not declared. Should it be static?
sound/soc/atmel/tse850-pcm5142.c:132:5: warning: symbol 'tse850_put_mix' was not declared. Should it be static?
sound/soc/atmel/tse850-pcm5142.c:154:5: warning: symbol 'tse850_get_ana' was not declared. Should it be static?
sound/soc/atmel/tse850-pcm5142.c:187:5: warning: symbol 'tse850_put_ana' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Mark Brown <broonie@kernel.org>