V4L/DVB: soc-camera: update comment

soc-camera no longer requires .set_crop() implementations to update their
argument. Update the commentary.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>