media: imx: imx7-media-csi: Simplify imx7_csi_video_init_format()

The imx7_csi_video_init_format() function instantiates a
v4l2_subdev_format on the stack, to only use the .format field of that
structure. Replace it with a v4l2_mbus_framefmt instance.

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>