fp-device: Unref the print once we've notified the progress

When we notify the enroll progress with a print, this needs to be unreffed
once we're done, but this only was happening in case of error.

Since it's not up to the callback function to free it, let's do it at the
end of the function.

As per this, clarify the docs for FpEnrollProgress marking it as transfer
none.

libfprint/fp-device.c

@@ -2344,6 +2344,8 @@ fpi_device_enroll_progress (FpDevice *device,
     }
   if (error)
     g_error_free (error);
+
+  g_clear_object (&print);
 }