[media] omap3isp: Don't try to locate external subdev for mem-to-mem pipelines

Memory-to-memory pipelines have no external subdev, we shouldn't try to
locate one and validate its configuration. The driver currently works by
chance due to another bug that results in failure to locate the external
subdev being ignored.

This gets rid of the "omap3isp omap3isp: can't find source, failing now"
error message in the kernel log when operating on a memory-to-memory
pipeline.

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>