V4L/DVB: soc-camera: prohibit S_CROP, if internal G_CROP has failed

There is no sense in trying to set cropping if we cannot get current one
from the host driver.

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