mtd: rawnand: Move function prototypes after struct declarations

Move nand_scan[_with_ids]() and nand_wait_ready() at the end of the
file where all function prototype lies. This will also allow us to get
rid 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>