media: imx: imx7-media-csi: Fix frame sizes enumeration

Enumeration of the minimum, maximum and step values for the image width
does not take hardware constraints into account. Fix it.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>