binman: Return failure when a usage() message is generated

The tool must return an error code when invalid arguments are provided,
otherwise binman has no way of knowing that anything went wrong.

Correct this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Fixes: fab430be2f ("tools: add mkeficapsule command for UEFI...")

tools/mkeficapsule.c

@@ -977,7 +977,7 @@ int main(int argc, char **argv)
 			exit(EXIT_SUCCESS);
 		default:
 			print_usage();
-			exit(EXIT_SUCCESS);
+			exit(EXIT_FAILURE);
 		}
 	}