ASoC: mediatek: mark symbols static where possible

We get 2 warnings about global functions without a declaration
in the ASoC mediatek module when building with W=1:

sound/soc/mediatek/common/mtk-afe-fe-dai.c:26:5: warning: no previous prototype for 'mtk_regmap_update_bits' [-Wmissing-prototypes]
 int mtk_regmap_update_bits(struct regmap *map, int reg, unsigned int mask,
         ^
sound/soc/mediatek/common/mtk-afe-fe-dai.c:34:5: warning: no previous prototype for 'mtk_regmap_write' [-Wmissing-prototypes]
 int mtk_regmap_write(struct regmap *map, int reg, unsigned int val)

In fact, all of those functions are only used in the file in which
they are declared and don't need a declaration, but can be made static.

so this patch marks it 'static'.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>