V4L/DVB (12527): tw9910: do not lie about cropping abilities

The current tw9910 driver does not implement cropping correctly. Therefore, and
also because various rectangles in struct soc_camera_device are in user scale,
we cannot and shall not use rect_current as window location.

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