[media] omap3isp: Fix crash caused by subdevs now having a pointer to devnodes

Commit 3e0ec41c ("V4L: dynamically
allocate video_device nodes in subdevices") makes the
embedding video_device directly.

Fix accesses to the devnode accordingly.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>