[media] omap3isp: Don't ignore failure to locate external subdev

A failure to locate the external subdev for a non memory-to-memory
pipeline is a fatal error, don't ignore it.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@iki.fi>
Tested-by: Peter Meerwald <pmeerw@pmeerw.net>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>