ASoC: Remove export of snd_soc_dapm_stream_event()

The only thing that should ever be calling this is soc-core and that is
built as part of the same module so doesn't need the export.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>