Reject duplicate KMSK key programming

Adding the same KMSK key twice will cause the KMSK entry unable to be
revoked. Give error message and quit in this case, instead of allowing
the programming with warnings.

cli/mfg.c

@@ -1151,13 +1151,9 @@ static int kmsk_entry_add(int argc, char **argv)
 	}
 
 	if (switchtec_security_state_has_kmsk(&state, &kmsk)) {
-		if (!cfg.assume_yes)
-			fprintf(stderr,
-				"WARNING: the specified KMSK entry already exists on the device.\n"
-				"Writing duplicate KMSK entries could make your device unbootable!\n");
-		ret = ask_if_sure(cfg.assume_yes);
-		if (ret)
-			return ret;
+		fprintf(stderr,
+			"REJECTED: the specified KMSK entry already exists on the device!\n");
+		return -8;
 	}
 
 	if (state.secure_state == SWITCHTEC_INITIALIZED_SECURED &&