ASoC: wm_adsp: Attach buffers and streams together

The stream is created whilst the compressed stream is opened and a
buffer is created when the DSP powers up. It is necessary at a point
once both the DSP has powered up and the the stream has been opened to
connect a stream to a buffer on the DSP. This is done in the trigger
callback as this is after the DSP has been powered and obviously the
stream must be open. Note that whilst the connect is currently trivial
it is expected that this will get more complex when support for multiple
buffers/streams per DSP is added.

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