media: imx: imx7-media-csi: Zero format struct before calling .get_fmt()

The v4l2_subdev_format structure passed to the .get_fmt() subdev
operation in imx7_csi_video_validate_fmt() isn't zeroed, which can cause
undefined behaviour. Fix it.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Adam Ford <aford173@gmail.com> #imx8mm-beacon-kit
Acked-by: Rui Miguel Silva <rmfrfs@gmail.com>
Tested-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>