ALSA: line6: Reorganize card resource handling

This is a fairly big rewrite regarding the card resource management in
line6 drivers:

- The card creation is moved into line6_probe().  This adds the global
  destructor to private_free, so that each driver doesn't have to call
  it any longer.

- The USB disconnect callback handles the card release, thus each
  driver needs to concentrate on only its own resources.  No need to
  snd_card_*() call in the destructor.

- Fix the potential stall in disconnection by removing
  snd_card_free().   It's replaced with snd_card_free_when_closed()
  for asynchronous release.

- The only remaining operation for the card in each driver is the call
  of snd_card_register().  All the rest are dealt in the common module
  by itself.

- These ended up with removal of audio.[ch] as a result of a reduction
  of one layer.  Each driver just needs to call line6_probe().

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