[media] omap3isp: Don't fail streamon when the sensor doesn't implement s_stream

The code handles subdevs with no s_stream operation correctly, but
returns -ENOIOCTLCMD by mistake if the first subdev in the chain has no
s_stream operation. Return 0 in that case.

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