Don't return directly if we just have a command code.

Mifare DESFire EV1 with authentication with AES or 3K3DES keys will require to
perform CMACing on each frame, even those with just a command.  So it does not
break anything as of today, but would be a problem later on.

libfreefare/mifare_desfire_authenticate.c

@@ -141,10 +141,6 @@ mifare_cryto_preprocess_data (MifareTag tag, void *data, size_t *nbytes, off_t o
     if (!key)
 	return data;
 
-    // Return directly if we just have a command code.
-    if (1 == *nbytes)
-	return data;
-
     switch (communication_settings & MDCM_MASK) {
     case MDCM_PLAIN:
 	break;