mtd: rawnand: Rename the manufacturer structure

It is currently called nand_manufacturer but could actually be called
nand_manufacturer_desc, like its instances, so that the former name is
left unused for now.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/linux-mtd/20200529111322.7184-5-miquel.raynal@bootlin.com