mtd: nand: Fix nanddev_neraseblocks()

nanddev_neraseblocks() currently returns the number pages per LUN
instead of the total number of eraseblocks.

Fixes: 9c3736a3

 ("mtd: nand: Add core infrastructure to deal with NAND devices")
Cc: <stable@vger.kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>