media: rkisp1: Zero v4l2_subdev_format fields in when validating links

The local sd_fmt variable in rkisp1_capture_link_validate() has
uninitialized fields, which causes random failures when calling the
subdev .get_fmt() operation. Fix it by initializing the variable when
declaring it, which zeros all other fields.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Dafna Hirschfeld <dafna@fastmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>