[media] ov772x: try_fmt must not default to the current format

If the requested format isn't supported, return a fixed default format
instead of the current format.

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