mtd: rawnand: Get rid of chip->chipsize

The target size can now be returned by nanddev_get_targetsize(). Get
rid of the chip->chipsize field and use this helper instead.

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