media: staging: media: imx: imx7-media-csi: Define macro for default mbus code

Define a macro for the default media bus code and use it through the
driver to replace a hardcoded value and a dynamic query from the
pixel_formats table.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Rui Miguel Silva <rmfrfs@gmail.com>
Tested-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>