mtd: nand: convert to nand_get_flash_node()

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

---8<----
virtual patch

@@
struct nand_chip c;
struct nand_chip *cp;
@@
(
-(cp)->flash_node
+nand_get_flash_node(cp)
|
-(c).flash_node
+nand_get_flash_node(&c)
)
---8<----

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>