nand: Fix access to last block in NAND devices

Currently, the last block of NAND devices can't be accessed. This patch
fixes this issue by correcting the boundary checking (off-by-one error).

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Scott Wood <scottwood@freescale.com>
Cc: Wolfgang Denk <wd@denx.de>