ASoC: rk3399_gru_sound: Fix non static symbol warning

Fixes the following sparse warning:

sound/soc/rockchip/rk3399_gru_sound.c:41:14: warning:
 symbol 'rt5514_dmic_delay' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Mark Brown <broonie@kernel.org>