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