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