serial: fix cleanup of flash function (rh #591728)

The flash function could be called when the port was closed, and since
the flash function would only be canceled when the port was open,
it could trigger after the port object was destroyed.

src/mm-errors.h

@@ -25,6 +25,7 @@ enum {
     MM_SERIAL_ERROR_RESPONSE_TIMEOUT = 2,
     MM_SERIAL_ERROR_OPEN_FAILED_NO_DEVICE = 3,
     MM_SERIAL_ERROR_FLASH_FAILED = 4,
+    MM_SERIAL_ERROR_NOT_OPEN = 5,
 };
 
 #define MM_SERIAL_ERROR (mm_serial_error_quark ())