Add a const version of the MediaObject::dev() method to be able to
retrieve a pointer to a const MediaDevice from a constant instance of
a MediaObject sub-class.
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>