ALSA: line6: Rearrange PCM structure

Introduce a new line6_pcm_stream structure and group individual
fields of snd_line6_pcm struct to playback and capture groups.

This patch itself just does rename and nothing else.  More
meaningful cleanups based on these fields shuffling will follow.

Tested-by: Chris Rorvick <chris@rorvick.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>