staging: line6: Move control endpoints to properties

The device type can now be used to determine the addresses of the
control 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>