[media] v4l: omap4iss: Make omap4iss_isp_subclk_(en|dis)able() functions void

The functions always succeed, there's no need to return an error value.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>