mtd: atmel_nand: fix wrong use of 0 as NULL

Fixing this error:
atmel_nand.c:718:20: warning: Using plain integer as NULL pointer

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Artem Bityutskiy <dedekind1@gmail.com>