mtd: spi-nor: convert to spi_nor_{get, set}_flash_node()

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

---8<----
virtual patch

@@
struct spi_nor b;
struct spi_nor *c;
expression d;
@@
(
-(b).flash_node = (d)
+spi_nor_set_flash_node(&b, d)
|
-(c)->flash_node = (d)
+spi_nor_set_flash_node(c, d)
)
---8<----

And a manual conversion for the one use of spi_nor_get_flash_node().

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