mtd: rawnand: fsl_upm: Remove unused mtd var

The mtd var in fun_wait_rnb() is now unused, let's get rid of it and
fix the warning resulting from this unused var.

Fixes: 50a487e7 ("mtd: rawnand: Pass a nand_chip object to chip->dev_ready()")
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200603134922.1352340-2-boris.brezillon@collabora.com