mtd: rawnand: Get rid of nand_flash_dev forward declation

nand_scan[with_ids]() have been moved at the end of the file. We can
now get rid of of the nand_flash_dev forward declaration.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>