mtd: nand: convert to nand_set_flash_node()

Used semantic patch with 'make coccicheck MODE=patch COCCI=script.cocci':

---8<----
virtual patch

@@
struct nand_chip *c;
struct device_node *d;
@@
-(c)->flash_node = (d)
+nand_set_flash_node(c, d)
---8<----

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>