mtd: rawnand: Reorganize code to avoid forward declarations

Avoid forward declaration of nand_get_device(), nand_do_write_oob()
and nand_update_bbt() by moving functions around.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Tested-by: Janusz Krzysztofik <jmkrzyszt@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>