mtd: concat: set the return lengths properly

In concat_read_oob both retlen and oobretlen should be updated.
concat_write_oob previously only (improperly) updated retlen.

Signed-off-by: Niklas Cassel <[e-mail]>
Signed-off-by: Brian Norris <[e-mail]>

drivers/mtd/mtdconcat.c

···
 			devops.len = subdev->size - to;
 
 		err = mtd_write_oob(subdev, to, &devops);
-		ops->retlen += devops.oobretlen;
+		ops->retlen += devops.retlen;
+		ops->oobretlen += devops.oobretlen;
 		if (err)
 			return err;