[media] V4L: cosmetic clean up

Improve the use of the WARN_ON() macro and use a local variable, instead
of reduntantly dereferencing a pointer in v4l2-dev.c

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>