media: staging: media: imx: imx7-media-csi: Simplify default mbus code in try_fmt

When trying a format on the subdev sink pad, if the requested media bus
code isn't supported, fallback to the default with a simpler logic.

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>