drm: rcar-du: Fix the return value in case of error in 'rcar_du_crtc_set_crc_source()'

We return 0 unconditionally in 'rcar_du_crtc_set_crc_source()'.
However, 'ret' is set to some error codes if some function calls fail.

Return 'ret' instead to propagate the error code.

Fixes: 47a52d02 ("media: drm: rcar-du: Add support for CRC computation")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>