ASoC: Intel: Make sure sst_byt_notify_work is run before freeing the stream

There is possibility that sst_byt_notify_work() is run after
sst_byt_stream_free() which can lead to a kernel crash since struct
sst_byt_stream is freed at that point.

Fix this by calling cancel_work_sync() for the stream notify work before
freeing the stream structure.

Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>