ALSA: line6: Add hwdep interface to access the POD control messages

We must do it this way, because e.g. POD X3 won't play any sound unless
the host listens on the bulk EP, so we cannot export it only via libusb.

The driver currently doesn't use the bulk EP messages in other way,
in future it could e.g. sense/modify volume(s).

Signed-off-by: Andrej Krutak <dev@andree.sk>
Signed-off-by: Takashi Iwai <tiwai@suse.de>