mtd: rawnand: Initialize the nand_device object

In order to use some of the nanddev_xxx() helpers, we need to
initialize the nand_device object embedded in nand_chip using
nanddev_init(). This requires implementing nand_ops.

We also drop useless mtd->xxx initialization when they're already taken
case of by nanddev_init().

Signed-off-by: Boris Brezillon <bbrezillon@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>