ALSA: line6: Drop superfluous snd_device for PCM

Instead of handling the card-specific resource in snd_device, attach
it into pcm->private_data and release it directly in private_free.
This simplifies the code and structure.

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