ALSA: line6: Drop superfluous snd_device for rawmidi

Like the previous fix for PCM, attach the card-specific resource into
rawmidi->private_data instead of handling in a snd_device object.
This simplifies the code and structure.

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