staging: line6: Move audio endpoints to properties

The device type can now be used to determine the addresses of the
audio endpoints for the interface.  Drop the conditional logic and make
these values properties.

Signed-off-by: Chris Rorvick <chris@rorvick.com>
Reviewed-by: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>