mtd: nand: Move nand_device forward declaration to the top

This structure might be used earlier in this file, let's move the
forward declaration at the top.
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/linux-mtd/20200529002517.3546-10-miquel.raynal@bootlin.com

include/linux/mtd/nand.h

@@ -12,6 +12,8 @@
 
 #include <linux/mtd/mtd.h>
 
+struct nand_device;
+
 /**
  * struct nand_memory_organization - Memory organization structure
  * @bits_per_cell: number of bits per NAND cell
@@ -133,8 +135,6 @@ struct nand_bbt {
 	unsigned long *cache;
 };
 
-struct nand_device;
-
 /**
  * struct nand_ops - NAND operations
  * @erase: erase a specific block. No need to check if the block is bad before