V4L/DVB (10095): The failure of set_fmt is solved in tw9910

priv->scale is checked in start_capture.
Therefore, it should be NULL if failing in set_fmt.
This patch resolve this problem.

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