ALSA: asoc: core - merge structs snd_soc_codec_dai and snd_soc_cpu_dai.

This patch series merges struct snd_soc_codec_dai and struct
snd_soc_cpu_dai into struct snd_soc_dai in preparation for further
ASoC v2 patches.

This merger removes duplication in both DAI structures and simplifies
the API for other users.

Signed-off-by: Liam Girdwood <lg@opensource.wolfsonmicro.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>