media: renesas: vsp1: Print debug message to diagnose validation failure

When formats don't match between a subdev and a connected video device,
starting streaming returns an error without giving the user any
indication as to what went wrong. To help debugging pipeline
misconfigurations, add a debug message that indicates the cause of the
failure.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>