crypto: atmel-aes - Add NIST 800-38A's zero length cryptlen constraint

NIST 800-38A requires for the ECB, CBC, CFB, OFB and CTR modes that
the plaintext and ciphertext to have a positive integer length.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>