media: imx: imx7-media-csi: Validate capture format in .link_validate()

Move capture format validation from imx7_csi_configure() to
.link_validate(). This simplifies error handling at stream on time, as
imx7_csi_configure) now becomes a void function.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rui Miguel Silva <rmfrfs@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>