crypto: atmel-tdes - Add FIPS81's zero length cryptlen constraint

FIPS81 requires for the ECB, CBC, CFB, and OFB modes that the
plaintext and ciphertext to have a positive integer length.
Add this constraint and just return 0 for a zero length cryptlen.

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