ASoC: wm_adsp: Detach compressed stream on free

If someone powers down the DSP core (through routing changes
say) whilst a compressed record is in progress we can end up
using a freed pointer to the buffer object. When a compressed
audio stream is triggered we attach it to a buffer on a physical
DSP. This patch adds a detach of the buffer from the stream when
the stream is freed or when the DSP is powered down which avoids
the situation where we use a buffer when it is no longer valid.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>