ASoC: Remove export of s3c_pcm_dai

We don't need to export s3c_pcm_dai after multi-component patch.
Thus remove export of s3c_pcm_dai and make it static.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>