ASoC: imx-card: Make some symbols static

The sparse tool complains as follows:

sound/soc/fsl/imx-card.c:121:27: warning:
 symbol 'ak4458_fs_mul' was not declared. Should it be static?
sound/soc/fsl/imx-card.c:138:31: warning:
 symbol 'ak4458_tdm_fs_mul' was not declared. Should it be static?
sound/soc/fsl/imx-card.c:149:27: warning:
 symbol 'ak4497_fs_mul' was not declared. Should it be static?
sound/soc/fsl/imx-card.c:166:27: warning:
 symbol 'ak5558_fs_mul' was not declared. Should it be static?
sound/soc/fsl/imx-card.c:180:31: warning:
 symbol 'ak5558_tdm_fs_mul' was not declared. Should it be static?

Those symbols are not used outside of imx-card.c, so marks
them static.

Fixes: aa736700 ("ASoC: imx-card: Add imx-card machine driver")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Message-Id: <20210524133553.2366502-1-weiyongjun1@huawei.com>
Signed-off-by: Mark Brown <broonie@sirena.org.uk>