mtd: Avoid magic values

Nvmem producer config ID "-1" is actually defined, so use the definition
rather than hardcoding it with a magic value.

Suggested-by: Michael Walle <michael@walle.cc>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Michael Walle <michael@walle.cc>
Link: https://lore.kernel.org/linux-mtd/20230307192536.470997-1-miquel.raynal@bootlin.com

drivers/mtd/mtdcore.c

@@ -519,7 +519,7 @@ static int mtd_nvmem_add(struct mtd_info *mtd)
 	struct device_node *node = mtd_get_of_node(mtd);
 	struct nvmem_config config = {};
 
-	config.id = -1;
+	config.id = NVMEM_DEVID_NONE;
 	config.dev = &mtd->dev;
 	config.name = dev_name(&mtd->dev);
 	config.owner = THIS_MODULE;