ASoC: hisilicon: localize functions without external linkage

A driver for hi6210 sound interface on hi6220 boards includes some
functions which has no external linkage. These functions should have
static qualifier.

This commit adds the qualifier to localize the functions. This issue is
detected by sparse:

hi6210-i2s.c:100:5: warning: symbol 'hi6210_i2s_startup' was not declared. Should it be static?
hi6210-i2s.c:178:6: warning: symbol 'hi6210_i2s_shutdown' was not declared. Should it be static?
hi6210-i2s.c:527:27: warning: symbol 'hi6210_i2s_dai_init' was not declared. Should it be static?

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