mtd: atmel_nand: avoid to report an error when lookup table offset is 0.

Before this patch, we assume the whole ROM code are mapping to memory. So
it is wrong if the lookup table offset is 0.

After this patch, we can map only the lookup table of ROM code to memory
intead of the whole ROM code (about 1M). In this case, one lookup table
offset can be 0.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>