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>

Author: ambarus

drivers/crypto/atmel-tdes.c

@@ -684,6 +684,9 @@ static int atmel_tdes_crypt(struct skcipher_request *req, unsigned long mode)
 	struct atmel_tdes_reqctx *rctx = skcipher_request_ctx(req);
 	struct device *dev = ctx->dd->dev;
 
+	if (!req->cryptlen)
+		return 0;
+
 	switch (mode & TDES_FLAGS_OPMODE_MASK) {
 	case TDES_FLAGS_CFB8:
 		if (!IS_ALIGNED(req->cryptlen, CFB8_BLOCK_SIZE)) {