[media] media: Properly handle link flags in link setup, link notify callback

The link flags were not properly handled in __media_entity_setup_link
which could lead to link_notify callback to be called when the flags are
not modified. Fix this.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>