staging: line6: Move altsetting to properties

The device type can now be used to determine the altsetting for the
interface.  Drop the conditional logic and make this value a property.

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>