mtd: rawnand: Get rid of chip->numchips

The same information is provided by nanddev_ntargets().

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>