ASoC: rt5616: add static qualifier for file local symbols

Sparse reports below warnings.

rt5616.c:1270:24: warning: symbol 'rt5616_aif_dai_ops' was not declared. Should it be static?
rt5616.c:1277:27: warning: symbol 'rt5616_dai' was not declared. Should it be static?

These two symbols are just used inner the file, thus it's better to add
static qualifier.

This commit adds it.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Mark Brown <broonie@kernel.org>