ASoC: cros_ec_codec: Make symbol 'cros_ec_dai' static

Fixes the following sparse warning:

sound/soc/codecs/cros_ec_codec.c:209:27: warning:
 symbol 'cros_ec_dai' was not declared. Should it be static?

Fixes: b291f42a ("ASoC: cros_ec_codec: Add codec driver for Cros EC")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Mark Brown <broonie@kernel.org>