ASoC: codec2codec: remove ephemeral variables

Now that codec to codec links struct snd_soc_pcm_runtime have lasting pcm
and substreams, let's use them. Alsa allocate and keep the
struct snd_pcm_runtime as long as the link is powered.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://lore.kernel.org/r/20190725165949.29699-6-jbrunet@baylibre.com
Signed-off-by: Mark Brown <broonie@kernel.org>