Fix warning in env_nand.c if compiled for DaVinci Schmoogie

Fix warnings

nv_nand.c: In function 'saveenv':
env_nand.c:200: warning: passing argument 3 of 'nand_write' from incompatible pointer type
env_nand.c: In function 'env_relocate_spec':
env_nand.c:275: warning: passing argument 3 of 'nand_read' from incompatible pointer type

if compiled for davinci_schmoogie_config.

Signed-off-by: Dirk Behme <dirk.behme@gmail.com>
Ack by: Sergey Kubushyn <ksi@koi8.net>